Question

takes two small nuclei and combines them into a larger nucleus
fusion
fission

Explanation:

In nuclear physics (a subfield of Physics, under Natural Science), nuclear fusion is defined as the process where two small atomic nuclei combine to form a larger nucleus. Fission, on the other hand, is the splitting of a large nucleus into smaller ones. So the description "Takes two small nuclei and combines them into a larger nucleus" matches fusion.

Answer:

fusion