Answer:
A heavier nucleus
Step-by-step explanation:
Nuclear fusion is the process in which two (or more) light nucleui fuse together combining into a heavier, more stable nucleus.
In this process, the total mass of the initial nuclei is slightly larger than the mass of the final nucleus: therefore, part of the initial mass has been transformed into energy, according to Einstein's famous equivalence
where m is the mass change and c is the speed of light. Given the huge value of c, this equation shows that a huge amount of energy is released for every process of nuclear fusion.
An example of nuclear fusion is the fusion of two atoms of hydrogen into an atom of helium, a process that occurs inside stars.