ANS:C - changes in heat of reaction.

Hess's law of constant heat summation is not based on conservation of mass; rather, it is based on the principle of conservation of energy. Hess's law states that the enthalpy change of a reaction is independent of the pathway between the initial and final states. In other words, the total enthalpy change for a reaction is the same regardless of the route taken. Hess's law deals with changes in heat of reaction, specifically enthalpy changes. It allows us to calculate the overall enthalpy change of a reaction by considering the enthalpy changes of individual steps or reactions that make up the overall reaction. So, the correct option is: changes in heat of reaction.