If the total mass of the products of a reaction is less than the mass of the reactants, what must be true?

A nuclear reaction has taken place.

A chemical reaction has taken place.

Nuclear fusion has occurred.

Nuclear fission has occurred.

The best answer is the third option. Nuclear fusion has occurred. It is possible for the mass of the reactants to be greater than the mass of the products during fusion reactions. The mass of the products is less than the mass of the reactants because some of the mass has been converted to energy. Fusion reaction happens when two or more nuclei combine or collide to form an element with a higher atomic number. In this process, some of the matter of the fusing nuclei is converted to energy.