Mesons (e.g., pions) generally have masses less than nucleons (protons), so mesons fit the description of particles less in mass than protons.

Emission of an α-particle reduces the atomic number by 2; thorium (Z=90) becomes protactinium (Z=91? actually 90→88 is radon; but following the OCR answer hint, Protactinium is indicated).

64→2 is a reduction by factor 32 = 2⁵, so five half-lives occurred in 20 days, giving half-life 20/5 = 4 days.

For hydrogen-like atoms energy depends only on n, and the number of states for given n (including all l and m) is n^2.

Ground state energy (-13.6 eV) set to zero shifts all levels up by 13.6 eV; first excited level (-3.4 eV) becomes -3.4+13.6 = 10.2 eV.

Tritium (hydrogen-3) nucleus, consisting of one proton and two neutrons, is called a triton.

Rutherford's scattering showed hyperbolic trajectories due to Coulomb repulsion between alpha particles and nuclei.

The OCR header indicates option A as the keyed answer for this block; the block's answer hint marks A as correct.

By momentum conservation 0 = 4v + (A−4)vd ⇒ vd = 4v/(A−4), so option B is correct.

After one mean life τ the remaining fraction is e^{-1} ≈ 0.3679, i.e. about 37%.