Why are very high temperatures and pressures required for fusion to occur?

a) to generate the neutrons that are needed to break the nuclei
b) to overcome the repulsion between the protons in the nuclei that join
c) to maintain the proper conditions to keep the chain reaction going
d) to keep the uranium fuel separate from the control rods

B) to overcome the repulsion between the protons in the nuclei that join

To overcome the repulsion between the protons in the nuclei that join.

Explanation:

In nuclear fusion reaction two small nuclei undergoes combination to give a larger nuclei with the emission of large amount of energy and some other subatomic nuclear particles.

Now when the two nuclei approach each other there is a repulsion between the nuclei of the two atoms, so in order to overcome that repulsion we have to provide high temperature and pressure conditions.

There is no relation between high temperature and proper conditions to keep chain reaction going on or separation of uranium from control rods.

In nuclear fission the nuclei break.