Option C

Nuclear fusion

Explanation :

A hydrogen bomb, also known as a thermonuclear bomb, uses the principle of nuclear fusion. In a fusion reaction, two lighter atomic nuclei combine to form a heavier nucleus, and a substantial amount of energy is released in the process. In the case of a hydrogen bomb, isotopes of hydrogen (such as deuterium and tritium) fuse together to form helium, releasing a large amount of energy.

It's worth noting that a hydrogen bomb usually involves a two-stage process. The first stage is a fission bomb (like those used in Hiroshima and Nagasaki) that creates the conditions necessary for the fusion reaction in the second stage. Despite this, the majority of the energy in a hydrogen bomb comes from fusion, which is why it is categorized as a fusion weapon.