Question

In what state must matter exist for fusion reactions to take place?

Answer 1

think of the sun, for it is a fusion engine.

Answer 2

Depends what you mean. Fusion takes place at a very low rate under any conditions at all. There are hydrogens fusing to deuterium in the Earth's ocean right now, I expect, either through exceedingly rare tunneling of one proton into another in momentarily close arrangements, the impact of cosmic rays, et cetera.

But if what you mean is a significant, measureable rate of fusion, then generally that requires extremely high temperatures, in the millions of degrees, and under those conditions all matter exists as a plasma (gas of nuclei and electrons).

Answer 3

plasma