Which one of the following is the energy source of stars ?

The primary energy source of stars is nuclear fusion. In the core of a star, extreme temperature and pressure conditions allow light atomic nuclei, typically hydrogen, to overcome electrostatic repulsion and fuse together to form heavier nuclei, such as helium.

During this fusion process, a small amount of mass is lost (mass defect) and converted into a tremendous amount of energy, as described by Albert Einstein's mass-energy equivalence equation, E = mc². In contrast, nuclear fission involves the splitting of heavy nuclei (like Uranium) and is the process used in terrestrial nuclear power plants. Chemical and photochemical reactions involve the rearrangement of electrons and produce significantly less energy than nuclear reactions, making them insufficient to power a star.