Moseley's law relates X-ray frequencies to atomic number and led to reordering of elements like potassium and argon by atomic number.

The molecular size is generally much smaller than the separation between molecules in a gas or liquid, so option D is correct.

Photons interacting with matter (photoelectric effect) remove electrons from atoms.

Band structure arises because many electrons obey the Pauli exclusion principle, filling allowed energy levels and forming bands in a periodic potential.

Length contraction predicts the length along motion tends to zero as speed approaches c, so at v = c it would contract to zero.

One becquerel (1 Bq) is defined as one nuclear decay per second.

Gamma rays are uncharged photons and do not produce definite ionization tracks like charged particles.

Alpha particles have a much higher biological effectiveness (RBE) than X-rays; typically their RBE is about 20, so they are ~20 times more damaging for the same absorbed dose.

Ernest Rutherford led experiments revealing the nuclear structure of the atom, fundamentally changing the model of the atom.

Putting p=2 in the Rydberg formula yields the Balmer series, which corresponds to transitions terminating at n=2.