ANS:A - fusion

The energy of the Sun arises mainly from fusion reactions. The Sun primarily generates its energy through nuclear fusion, where hydrogen nuclei combine to form helium nuclei in a process known as thermonuclear fusion. This process occurs in the Sun's core where temperatures and pressures are extremely high. The fusion of hydrogen nuclei releases vast amounts of energy in the form of electromagnetic radiation, mainly in the form of visible light and heat. In contrast, fission reactions involve the splitting of heavy atomic nuclei into smaller nuclei, releasing energy in the process. Fission reactions occur in nuclear reactors and certain natural processes, but they are not the primary source of energy for the Sun. Combustion, on the other hand, involves chemical reactions between a fuel and an oxidizing agent, typically involving carbon-based compounds such as hydrocarbons. Combustion processes are important on Earth for generating energy, but they do not occur in the Sun, which primarily relies on nuclear fusion for its energy production. Therefore, the energy of the Sun arises mainly from fusion reactions.