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Nuclear fusion is the process that powers the Sun and other stars. Option B is correct: Nuclear fusion is the process in which two light atomic nuclei, typically hydrogen isotopes (deuterium and tritium), combine under extremely high temperature and pressure to form a heavier nucleus (helium), releasing enormous energy. Fusion releases much more energy per unit mass than fission. Option A is incorrect: Splitting of a heavy nucleus describes nuclear fission, which is the opposite process. Fission is used in nuclear power plants and atomic bombs. Option C is incorrect: Emission of neutrons from a nucleus can occur during fission or in some radioactive decays (neutron emission), but it does not define nuclear fusion. Option D is incorrect: Decay of a radioactive nucleus is radioactive decay, which involves alpha, beta, or gamma emission from an unstable nucleus. It is a different nuclear process from fusion. The hydrogen bomb uses uncontrolled nuclear fusion triggered by a fission bomb.
