Which best describes nuclear fusion? A nucleus spontaneously splits and absorbs energy.
Two nuclei spontaneously combine and absorb energy.
A nucleus collides with a neutron and splits, releasing energy.
Nuclei combine to form a heavier nucleus, releasing energy.

The statement that best describes nuclear fusion is;

Nuclei combine to form a heavier nucleus, releasing energy

Explanation:

In nuclear fusion, we have the reaction of the nuclei of two or more atoms coming together (combining) to form heavier elements and subatomic particles such as protons and neutrons accompanied by the release or absorption in energy depending on the difference between the mass of the reactants and the products

Some nuclear fusion reaction require an input of energy and such reactions are therefore not spontaneous

The best option is nuclei (two or more nuclei) combine to form a heavier nucleus, releasing energy.