Positronium has reduced mass μ = m/2, so orbital speeds scale with μ and are half those in hydrogen, giving factor 1/2.

Alpha emission lowers mass by 4; beta emission does not change mass number, so net change is -4.

A β-ray is an electron produced when a neutron decays into a proton inside the nucleus, so option C is correct.

Hyperfine structure arises from interactions involving the nucleus, such as nuclear magnetic spin coupling, so option D is correct.

Mean life τ = T1/2 / ln2 = 1600 / 0.693 ≈ 2309 years, so option C is correct.

By momentum conservation: 4·V = (A-4)·v_recoil, so v_recoil = 4V/(A-4).

For hydrogen-like species the line energy scales with Z^2; singly ionized helium (He+, Z=2) produces the shortest wavelength among the listed hydrogen-like ions; option C is correct.

X-rays arise from inner-shell transitions in multi-electron atoms; hydrogen has only one electron so such high-energy X-ray transitions do not occur.

This 'whole-number' hypothesis was proposed by William Prout, known as Prout's hypothesis.

The described phenomenon is the photoelectric effect, where photons eject electrons from a metal surface.