James Chadwick discovered the neutron in 1932, corresponding to option A.

A common classification lists three broad types of reactors (e.g., research, power, and propulsion reactors), hence three is often the expected answer in basic MCQs.

If the proton's velocity is parallel to the magnetic field, the magnetic force q(v × B) is zero, so it will not be deflected.

X-rays are electromagnetic radiation and travel at the speed of light, approximately 3 × 10^8 m/s.

Radon is produced in the Earth's crust and seeps from the ground into buildings, so 'Ground' is the source.

After two mean lives mass = m0 e^{-2} = 10×e^{-2} ≈ 10×0.1353 ≈ 1.35 gm.

The described concept is time dilation, where moving clocks are observed to run slower.

A neutral atom has equal numbers of protons and electrons to balance charge.

Moseley's law relates characteristic X-ray frequencies to the atomic number of elements, so atomic number is correct.

γ-rays are weakly ionizing and produce sparse, often thin and discontinuous tracks in a cloud chamber.