Pair production occurs when gamma-ray energy exceeds the threshold 1.02 MeV near a nucleus, matching option A per the OCR answer key.

Three main processes (photoelectric effect, Compton scattering, and pair production) account for X- and gamma-ray absorption, corresponding to option B.

A meson is composed of a quark–antiquark pair, so Meson is the correct option.

A dosimeter measures exposure to ionizing radiation and is used for radiation safety monitoring.

Alpha decay half-lives are predicted using barrier tunneling approximations such as the WKB method, option (a).

Ernest Rutherford is credited with the first artificial nuclear transmutation, so option B is correct.

10.2 eV corresponds to the Lyman-α transition with wavelength ≈ 1216 Å, so option ~1217 Å is correct.

Radioactive decay follows an exponential law, so activity decreases exponentially; the OCR shows option D marked.

X-rays in an X-ray tube are produced when high-energy electrons bombard a target and decelerate (bremsstrahlung) or cause characteristic emissions.

Beyond iron, binding energy per nucleon generally decreases as mass number increases, so option B is the typical answer.