ANS:C - a combination of both nuclear fission & fusion.

The phenomenon occurring during the explosion of a hydrogen bomb is nuclear fusion. Explanation: A hydrogen bomb, also known as a thermonuclear bomb or fusion bomb, relies on the process of nuclear fusion to release energy. In a hydrogen bomb, isotopes of hydrogen, such as deuterium and tritium, undergo fusion reactions under extremely high temperatures and pressures, similar to those found in the core of stars. During the explosion of a hydrogen bomb, the fusion of hydrogen isotopes into helium releases an enormous amount of energy, much greater than that released in a typical atomic bomb, which relies on nuclear fission. While nuclear fission may also occur as part of the hydrogen bomb detonation process (triggered by the intense heat and pressure generated by the fusion reaction), the primary mechanism driving the explosion is nuclear fusion. Therefore, the correct phenomenon occurring during the explosion of a hydrogen bomb is nuclear fusion.