Answer: B. low mass nuclei are combined into heavier ones, releasing energy in the process
Explanation:
Nuclear fusion is a process which involves the conversion of two small nuclei to form a heavy nuclei along with release of energy.
For Example: [tex]4_1^1\textrm{H}\rightarrow _2^4\textrm{He}+2_{+1}^0\textrm{e}+\text{energy}[/tex]
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.
For Example: [tex]_{92}^{235}\textrm{U}+_0^1\textrm{n}\rightarrow _{56}^{143}Ba+_{36}^{90}Kr+3_0^1\textrm{n}[/tex]
Thus if a heavier nuclei is being formed from two smaller nuclei releasing energy this is a nuclear fusion reaction.