On excitation to higher n, kinetic energy (positive) decreases while potential energy (negative) becomes less negative (i.e., increases).

Pair production requires photon energy at or above the threshold 2 m0 c^2, so it occurs for energies greater than or equal to that value.

Answer taken from block answer hint; header indicates option A as correct in the OCR metadata.

A free neutron undergoes beta decay to form a proton and an electron (beta particle), so a proton and an electron is correct.

Classical wave theory fails to explain photoelectric observations such as threshold frequency and instantaneous emission, so 'Any of them' is appropriate.

Sommerfeld's model gives one circular orbit (maximum angular momentum) and n−1 elliptical orbits; for n=3 that yields one circular and two elliptical orbits.

Leptons do not participate in the strong nuclear interaction, unlike hadrons and nucleons, so leptons is correct.

Na-23 nucleus has 11 protons and 12 neutrons and no electrons, so option A is correct.

Absorbed dose is proportional to deposited energy; to double the dose the energy delivered must be doubled.

Solid-state radiation detectors are typically reverse-biased p–n junctions (semiconductor diodes) used to collect charge from ionizing events.