When the products of a reaction are hotter than the reactants: the reaction is exothermic. the reaction is endothermic. the reactants lost internal energy. the change in enthalpy is positive.

Option (a) is the correct answer.

Explanation:

A chemical reaction in which energy is released is known as an exothermic reaction.

For example,

In an exothermic reaction, energy of products is less than the energy of reactants. As heat is released that is why, formation of products become hotter.

On the other hand, a chemical reaction in which energy is absorbed by reactant molecules is known as an endothermic reaction.

For example,

In an endothermic reaction, energy of products is more than the energy of reactants. As heat is absorbed, so formation of products become colder.

Thus, we can conclude that when the products of a reaction are hotter than the reactants the reaction is exothermic.