Answer: D) nuclear fusion because an atomic nuclei combine to form a heavier nucleus
Step-by-step explanation:
Nuclear fusion is a process which involves the conversion of two small nuclei to form a heavy nuclei along with release of energy.
Nuclear fission is a process which involves the conversion of a heavier nuclei into two or more small and stable nuclei along with the release of energy.

As in the given reaction a heavier nuclei helium is being formed from combination of deuterium and tritium, this is a nuclear fusion reaction.