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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. The Sun generates energy by fusing hydrogen into helium at temperatures of about 15 million Kelvin. 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.
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