Nuclear fusion is a process where
a. low mass nuclei are combined into heavier ones, absorbing energy in the process
b. low mass nuclei are combined into heavier ones, releasing energy in the process
c. high mass nuclei are broken-up into lighter ones, absorbing energy in the process
d. high mass nuclei are broken-up into lighter ones, releasing energy in the process
0.) none of the above

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:

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:  

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