Question

The reaction occurs only at extremely high temperatures.
O Fusion
O Fission

Answer 1

fission

Step-by-step explanation:

Answer 2

fusion

To make fusion happen, the atoms of hydrogen must be heated to very high temperatures (100 million degrees) so they are ionized (forming a plasma) and have sufficient energy to fuse, and then be held together i.e. confined, long enough for fusion to occur. The sun and stars do this by gravity.

fission doesn't require high temperature