All electromagnetic waves, including X-rays and visible light, travel at the same speed c in vacuum.

Alpha particles are helium nuclei with positive charge, so the beam is of positively charged particles.

Stellar energy is produced primarily by nuclear fusion reactions (e.g., hydrogen fusion in stars).

Niels Bohr introduced quantized atomic energy levels in his model, making option B correct.

When a nucleus splits into two comparable fragments it is undergoing nuclear fission.

Spectrum can be described equivalently by wavelength, frequency, or photon energy, so 'All of these' is correct.

Radioactivity involves the decay of unstable nuclei, so 'decay of nucleus' best describes the phenomenon.

The emission of particles and radiation by unstable nuclides is termed radioactivity.

The OCR header indicates option A as the provided correct numerical value for the given momentum.

In a body-centered cubic lattice the distance from body center to a corner atom is (sqrt(3)/2)·a ≈ 0.866·4.225 ≈ 3.66 Å, so option A is correct.