Nuclear fusion differs from nuclear fission because nuclear fusion reactions
(1) form heavier isotopes from lighter isotopes
(2) form lighter isotopes from heavier isotopes
(3) convert mass to energy
(4) convert energy to mass

Nuclear fusion is the forceful collision of two nuclei to make a larger one
So the answer is (1)
(2) Would be nuclear fission
For (3) and (4), you can't convert energy from anything that isn't another form of energy, and you can't convert it in to anything that isn't another from of energy

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