What is the best definition of the enthalpy of reaction, AH?

Amount of heat added to the system

Explanation:

Enthalpy of the reaction is the amount of heat added to the chemical reaction or system.

The Heat of Reaction (also known and Enthalpy of Reaction) is the change in the enthalpy of a chemical reaction that occurs at a constant pressure. It is a thermodynamic unit of measurement useful for calculating the amount of energy per mole either released or produced in a reaction.