What Do All Nuclear Fusion Reaction Have in Common

What do all nuclear fusion reaction have in common?

A) The splitting of a heavy atom to two atoms with lighter nuclei.
B) The combining of two nuclei to make a heavier product.
C) The addition of a proton or positron to an atom to form a new atom.
D) The release of energy when a large atom breaks down.
E) Nothing; there are too many types of fusion for any common factor.