Hydrogen fusion to helium in stars can proceed via either the proton-proton chain or the carbon (CNO) cycle, so option C is correct.

X-rays are uncharged electromagnetic radiation and are not deflected by electric or magnetic fields.

Typical textbook values cite the order of tens of kilograms as critical mass for some fissile materials, so option B is the common exam answer.

A cloud chamber operates with supersaturated vapour so that ionizing particles produce visible condensation trails.

Gamma rays (γ) are the most penetrating of the listed radiations and can penetrate thick steel, so option A is correct.

Half-life t1/2 = ln2 / λ ≈ 0.693 / (2.4×10^-8 s^-1) ≈ 2.9×10^7 s, matching option A.

The photon is a gauge boson that mediates the electromagnetic force, so 'Photon' is the correct gauge boson listed.

Energy band formation arises because many electron states must obey Pauli's principle, causing discrete levels to split into bands in solids.

For hydrogen-like ion energy = 13.6 × Z^2 = 13.6 × 9 = 122.4 eV for ground state ionization.

Moderators like heavy water are used in reactors to slow down fast neutrons effectively without capturing them.