Which of the following processes releases energy when atomic nuclei fuse?

The process that releases energy when atomic nuclei fuse is nuclear fusion. In nuclear fusion, light atomic nuclei combine to form a heavier nucleus. This process occurs under extreme conditions, typically found in stars like the Sun, where hydrogen nuclei fuse to create helium. During fusion, a small amount of mass is converted into energy as described by Einstein's equation, E=mc². The energy produced in fusion reactions is much greater than that released in fission or radioactive decay.

Nuclear fission, on the other hand, involves the splitting of a heavy nucleus into lighter nuclei, also releasing energy, but it is distinct from fusion. Radioactive decay is a process where an unstable atomic nucleus loses energy by emitting radiation; it doesn't involve fusion of nuclei. A chain reaction refers to a series of reactions where the products of one reaction propagate further reactions, which is commonly associated with fission rather than fusion.