Alpha particles are nuclear emissions, not radiations produced by electronic transitions in atoms.

Radioactive decay involves the nucleus emitting radiation (alpha, beta, gamma), so the nucleus emits at least one form of radiation.

A photocell includes an evacuated glass tube and a photoemissive cathode, so both (A) and (C) are parts.

The 1964 quark model proposed three quark types (up, down, strange).

Hadrons are composite particles (made of quarks), so they are not elementary, unlike photons, leptons, and quarks (which are elementary).

Activity 1/8 = (1/2)^3, so three half-lives; using C-14 half-life ≈5730 years gives 3×5730 = 17190 years.

Seven eighths decayed means one eighth remains = (1/2)^3, so time = 3 half-lives = 3 * 1622 = 4866 years.

In a chain reaction each generation multiplies neutrons, so the neutron number grows geometrically (option B).

Control rods are commonly made of cadmium (or boron) because they absorb neutrons and control the chain reaction.

Uranium-238 has a half-life of about 4.5 × 10^9 years, corresponding to option B.