Correct Answer - a
A fusion reaction is difficult to occur because positively charges nuclei repel each other. At very high temperatures of the order of `10^(6)` to `10^(7)K`, the nuclei may have sufficient energy to overcome the repulsive forces and fuse. It is for this reacons, fusion reactions are also called thermonuclear reactions. Hence, hydrogen can be fuesed to form helium at high temperature and high pressure.