Mass number is the sum of protons and neutrons, so it is sometimes equal (e.g., H-1) and usually more than the atomic number; D best matches.

Wolfgang Pauli proposed the neutrino to account for missing energy and momentum in beta decay.

Radioactive decay series are often shown on a Segrè (Segre) chart plotting neutron vs proton numbers.

Henri (rendered here as 'Henry') Becquerel discovered natural radioactivity.

The Balmer series includes visible spectral lines of hydrogen, whereas Lyman is ultraviolet and Paschen/Pfund are infrared.

Graphite (or heavy water) is commonly used as a moderator to slow neutrons in reactors; among the options, graphite is the moderator.

Twenty years is two half-lives, so remaining mass = 12 × (1/2)^2 = 3.0 g.

The neutrino is an electrically neutral, (to good approximation) massless particle compared with charged particles like electrons or protons.

Max Planck was awarded the Nobel Prize in Physics in 1918 for his work on quantum theory.

Remaining fraction 0.10 = (1/2)^(t/10) ⇒ t = 10·ln(0.1)/ln(0.5) ≈ 33.2 days, so approx 33 days (C).