In the Bohr model the orbit radius scales as the square of the principal quantum number, r_n ∝ n^2.

A positron is the antiparticle of the electron, so Electron (option B) is correct.

Pair production requires high-energy gamma photons to produce particle–antiparticle pairs.

X-ray emissions include both a continuous Bremsstrahlung background and discrete characteristic line spectra, so both types occur.

Crystalline solids have a regular three-dimensional repeating pattern of atoms or molecules.

Beta emission increases the atomic number by one; Po (Z=84) becomes At (Z=85), so astatine-218 (option C) is formed.

In β− decay a neutron converts to a proton and an electron, so the daughter nucleus has one more proton than the parent.

Stars generate energy primarily by fusion of light nuclei (e.g., hydrogen into helium) in their cores.

Stress can lead to changes in length, volume, or shape depending on its nature, so any of these may change.

Gamma rays are more penetrating but less ionizing than alpha and beta particles; thus their ionizing capability is less than both.