Energy generation in stars is mainly due to

The correct answer is Option (3) → fusion of light nuclei

Option (1) chemical reactions — Incorrect. The mass changes, and so the energies, in a chemical reaction are about a million times smaller than in a nuclear reaction. This is far too little for the energy output of a star.

Option (2) fission of heavy nuclei — Incorrect. Fission of heavy nuclei such as uranium-235 does release energy. But the fuel of a star is light nuclei. In the sun, hydrogen is burned into helium.

Option (3) fusion of light nuclei — Correct. In the hot interior of a star, light nuclei have enough kinetic energy to overcome their Coulomb repulsion. They fuse into a more tightly bound heavier nucleus and release energy. This thermonuclear fusion is the source of the energy of stars.

Option (4) fusion of heavy nuclei — Incorrect. Heavy nuclei already have a lower binding energy per nucleon than nuclei of middle mass. Joining them gives an even less tightly bound nucleus, so no energy is released.

NCERT (2026-27):
Nuclear Fusion – Energy Generation in Stars – “Thermonuclear fusion is the source of energy output in the interior of stars.”