The correct answer is A. Nuclear fission.
Nuclear fission is a nuclear reaction in which a large atomic nucleus, such as uranium-235, splits into two smaller nuclei. This process releases a large amount of energy, which can be used to generate electricity or to create an atomic bomb.
Nuclear fusion is a nuclear reaction in which 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. Fusion is the process that powers active stars, the hydrogen bomb and experimental devices examining fusion power for electrical generation.
A controlled chain reaction is a nuclear chain reaction in which the rate of the reaction is controlled so that the reaction does not run away. This is done by using a moderator to slow down the neutrons produced by the fission reaction. The moderator absorbs some of the neutrons, preventing them from causing further fission reactions. This allows the reaction to be controlled and used to generate electricity.
None of these is the correct answer.