Concept:The combining of two light atomic nuclei into a heavier nucleus, with energy released, is called nuclear fusion.Explanation:Radioactivity is the spontaneous breaking down of an unstable nucleus.Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei.A chain reaction is a self-sustaining sequence of fission reactions.In nuclear fusion, two light nuclei merge to form a heavier nuclide and release a large amount of energy.This matches the process described in the question.Answer:B. nuclear fusion