The correct answer is C. Nuclear fusion.
Nuclear fusion is the process that powers stars, including the Sun. It is a very powerful process that releases a tremendous amount of energy. In nuclear fusion, two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as the release of large amounts of energy.
Nuclear fission is the process of splitting a large atomic nucleus into two or more smaller nuclei. This process releases a large amount of energy, which is used in nuclear power plants.
Radioactivity is the process by which an unstable atomic nucleus loses energy by radiation. This radiation can be in the form of alpha particles, beta particles, or gamma rays.
Ionization is the process of removing an electron from an atom or molecule. This can be done by heating the atom or molecule, or by exposing it to radiation.