The correct answer is (A). Using the mirror formula \(\frac{1}{v} = \frac{1}{f} - \frac{1}{u}\) with \(u = -30\) cm and \(f = -15\) cm, we get \(\frac{1}{v} = -\frac{1}{15} + \frac{1}{30} = -\frac{1}{30}\), so \(v = -30\) cm. When an object is placed at \(2F\) of a concave mirror, the real image also forms at \(2F\) on the same side.

The correct answer is (B) 20 m/sec. At max height \(h = 20\) m, using \(v^2 = u^2 - 2gh\) (upward motion): \(0 = u^2 - 2(10)(20)\), so \(u^2 = 400\), \(u = 20\) m/s. Exam Tip: Max height formula: \(h = \frac{u^2}{2g}\). If \(h = 20\) m, then \(u = \sqrt{2 \times 10 \times 20} = 20\) m/s.

The correct answer is (B). Sodium carbonate (\(\text{Na}_2\text{CO}_3\)) is an inorganic salt because it contains no carbon-hydrogen bonds — its carbon is bonded only to oxygen in the carbonate ion. Ethanol, methane, and acetic acid all contain C-H bonds and are organic compounds.

The correct answer is (A). Since \(R = \frac{\rho L}{A}\), resistance is inversely proportional to cross-sectional area when material and length are the same. With area ratio \(A_1 : A_2 = 1:4\), the resistance ratio \(R_1 : R_2 = 4:1\), meaning the thinner wire has four times more resistance.

The correct answer is (A). The vas deferens is a muscular tube that transports mature sperm from the epididymis to the urethra during ejaculation through peristaltic contractions. Sperm production occurs in the testes, and seminal fluid is secreted by the seminal vesicles, not the vas deferens.

The correct answer is (D) 23. Mass number = Number of protons + Number of neutrons = \(11 + 12 = 23\). Mass number represents the total count of nucleons (protons and neutrons) in an atom's nucleus. Exam Tip: Mass number = \(A = p + n\). Atomic number = \(Z = p\). Remember: Mass number - Atomic number = neutrons.

The correct answer is (D) Thumb. In Fleming's Left-Hand Rule, the Thumb represents the direction of Force or Motion. The Index finger shows the magnetic field direction, and the Middle finger shows the current direction. This is the standard convention used in electric motors.

The correct answer is (B) Ribosome. Ribosomes are protein-synthesizing organelles that attach to the endoplasmic reticulum, giving it a 'rough' grainy appearance under the microscope. This is why it's called Rough ER (RER). Smooth ER lacks ribosomes.

The correct answer is (C) the solute particles do not settle down when left undisturbed. A stable solution is a homogeneous mixture where solute particles remain uniformly distributed throughout the solvent and do not separate or settle at the bottom. This uniform distribution is maintained over time without external disturbance.

The correct answer is (A) Sum of atomic masses of all atoms in the formula unit of an ionic compound. Formula Unit Mass (or Formula Mass) is the sum of atomic masses of all atoms present in one formula unit of an ionic compound. For example, in \(\text{NaCl}\), formula mass \(= 23 + 35.5 = 58.5\) u. This differs from molecular mass (used for covalent compounds) and molar mass (mass of one mole).

The correct answer is (A) Only a and b. Let's verify each statement using the Law of Constant Proportions: (a) Water (\(\text{H}_2\text{O}\)): \(\text{H}:\text{O} = 1:8\) by mass. Check: Atomic masses: \(\text{H}=1\), \(\text{O}=16\). In \(\text{H}_2\text{O}\): \(\text{H}_2 = 2\), \(\text{O} = 16\), ratio \(= 2:16 = 1:8\) ✓ Correct. (b) \(\text{CO}_2\): \(\text{C}:\text{O} = 3:8\) by mass. Check: \(\text{C} = 12\), \(\text{O}_2 = 32\). Ratio \(= 12:32 = 3:8\) ✓ Correct. (c) \(\text{NH}_3\): The statement says \(\text{N}:\text{H} = 14:1\), but actual is \(\text{N}:\text{H} = 14:3\) (\(\text{N}=14\), \(\text{H}=1\times3=3\)). The ratio \(14:1\) is incorrect ✗ Wrong. Therefore, only (a) and (b) follow the Law of Constant Proportions. Exam Tip: Always verify chemical ratios by calculating mass of each element separately.

The correct answer is (D) Prostate gland. The prostate gland is part of the male reproductive system. It secretes fluid that mixes with sperm to form semen. Vagina, Fallopian tube, and Uterus are parts of the female reproductive system, not male.

The correct answer is (D) Acid + Base → Salt + Water. Neutralisation is the chemical reaction between an acid and a base that produces a salt and water. When \(\text{HCl}\) (acid) reacts with \(\text{NaOH}\) (base), it forms \(\text{NaCl}\) (salt) and \(\text{H}_2\text{O}\) (water). This is an exothermic reaction that releases heat. Exam Tip: Remember the core neutralisation equation: Acid + Base → Salt + Water. This is the definition used in chemistry.

The correct answer is (C) Larger amplitude. Loudness of a sound is determined by its amplitude, not its frequency or wavelength. A louder sound has a larger amplitude (greater displacement of the medium). The frequency (and thus pitch) remains the same. Wavelength = speed/frequency, so it also remains constant if frequency is unchanged.

The correct answer is (A) Parthenium. Parthenium (Gajar-ka-patta or Congress grass) is an invasive weed that competes with crops for nutrients, water, and light. It reduces crop yield and is toxic to livestock. Removing it is beneficial. Wheat, Soybean, and Sunflower are cultivated crops, not weeds.

The correct answer is (B) Sweet potato. Sweet potato reproduces vegetatively through its tuberous roots (adventitious roots). These roots can detach from the parent plant and develop into new independent plants. This is a form of asexual reproduction called vegetative propagation. Gram, Potato, and Maize primarily propagate through seeds or are typically grown from other modified structures, not from roots that can regenerate. Exam Tip: Sweet potato roots = vegetative propagation. Most others = seed propagation.

The correct answer is (D) In the abdomen, on either side of the back bone. The kidneys are located in the posterior (back) abdomen on either side of the vertebral column (spine), between the T12 and L3 vertebrae. They sit just below the rib cage. They are NOT in the chest; they are retroperitoneal abdominal organs.

The correct answer is (B) Mass is constant and Weight is variable. Mass is the amount of matter in an object and remains constant regardless of location. Weight is the force of gravity acting on mass (\(W = mg\)) and varies depending on the gravitational field strength. On Earth vs. Moon, mass stays same but weight changes because gravity differs.

The correct answer is (B) Water pushes the swimmer forward. This demonstrates Newton's Third Law of Motion: for every action, there is an equal and opposite reaction. When the swimmer pushes water backward (action), the water exerts an equal force on the swimmer in the forward direction (reaction). The forward reaction force from the water is what propels the swimmer. Exam Tip: Action-Reaction pairs are equal in magnitude and opposite in direction. The swimmer pushes water back; water pushes swimmer forward.

The correct answer is (D) It increases the chance of survival under changing environmental conditions. Variation provides genetic diversity. When environmental conditions change, organisms with different traits have better chances of adapting and surviving (natural selection). Variation is the raw material for evolution and adaptation. Identical individuals would all face the same challenges and might all perish.