The OCR answer key indicates 550 barns for U-235 fission by slow neutrons, i.e. option B.

The OCR header gives option A as the answer for the neutron kinetic energy corresponding to the stated momentum.

The Sun's primary composition is hydrogen and helium, so option D is correct.

Neutrons produced in fission are fast and carry significant kinetic energy, so 'Very fast and more energetic' is correct.

The continuous spectrum is produced when incident electrons decelerate upon hitting the target, i.e., deceleration of impact electrons.

Gamma emission arises from the nucleus and does not involve atomic electronic structure, so option A is correct.

6 g of C-12 is 0.5 mol → 0.5×6.022×10^23 atoms ≈ 3.01×10^23 atoms; each atom has 6 neutrons → ≈1.81×10^24 neutrons, matching option C.

Cs-137 emits gamma (γ) rays, which are penetrating and suitable for thickness gauging, so option D is correct.

Half-life is an intrinsic property depending on the nature of the nuclide, not on external conditions in most cases.

C-12 (6 protons, 6 neutrons) and N-13 (7 protons, 6 neutrons) have the same number of neutrons (6), so they are isotones.