The source of the sun's heat is nuclear fusion.
Answer: B
Explanation
Nuclear fusion is defined as the reaction where two or more nuclei combine together to form two or more product nuclei.
In nuclear fusion reaction, the difference in the mass of the reactant nuclei converts into the energy released or absorbed along with the product nuclei.
Generally lighter elements like hydrogen and helium are more prone to undergo nuclear fusion and produce higher weight nuclei along with release of high amount of energy.
The lighter reacting elements can easily overcome the attractive forces between them and combine to form the product nuclei with release of energy.
In sun, the hydrogen and helium fusion chain reaction provides the necessary energy to emit heat radiations and remain as an active star.