Answer: 3.) atomic nuclei are united
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.

Thus if a heavier nuclei is being formed from two smaller nuclei, this is a nuclear fusion reaction.