The OCR answer key indicates option A; Hideki Yukawa predicted the meson and received the Nobel Prize.

Compton scattering increases the photon's wavelength, so the scattered wavelength is larger than the incident one.

Heavy water (a moderator) slows fast neutrons effectively through collisions with light nuclei, reducing their energy to thermal levels.

In the Rutherford experiment most alpha particles passed through the foil with negligible deflection, implying atoms are mostly empty space.

Bremsstrahlung power scales approximately as V^2 for a given current, so doubling voltage gives about four times the intensity.

Radioactivity arises from the disintegration (instability) of atomic nuclei.

Applying the Compton formula with θ=45° yields a small wavelength increase and a scattered photon energy ≈48.6 keV, matching option C.

Because of the binding energy, the mass of a bound nucleus is less than the sum of its free constituents, so the ratio is less than one.

Thorium is a radioactive metal and can exhibit magnetic properties in certain forms; the OCR has a marker next to Thorium.

There are four fundamental interactions: gravitational, electromagnetic, strong, and weak.