The OCR answer key lists option C; Dirac is credited in the text (Dirac predicted the positron theoretically).

If the commutator [A, B] = 0, the operators commute, so option C is correct.

Radiation produced when a charged particle exceeds light speed in a medium is the Cerenkov effect, so option D is correct.

Mean lifetime τ and half-life T1/2 are directly proportional (τ = T1/2 / ln2), so they scale proportionally.

Bohr's hypothesis postulates that angular momentum is quantized and can take only discrete values.

The OCR shows a selection mark adjacent to option A (Iodine-131), which is the common radioisotope used for thyroid treatment.

Melting involves the loss of long-range order in a solid, so the transition is from order to disorder.

1 eV ≈ 1.6×10^-19 J, so 1.6 eV ≈ 2.56×10^-19 J.

Atomic radius (~10^-10 m) divided by nuclear radius (~10^-15 m) gives a ratio around 10^5.

Fast reactors use a U-238 blanket to breed fissile material (e.g., Pu-239), so U-238 is the correct blanket material.