Basically, It's the total heat content of a system at constant temperature and pressure. so option b is correct.

Enthalpy:
Enthalpy is a thermodynamic property of a system that represents the heat exchanged with the surroundings at constant pressure. It is denoted by the symbol H and is expressed in units of energy (such as joules or calories).

Heat Exchanged in a Chemical Reaction:
When a chemical reaction occurs, it is often accompanied by a release or absorption of heat. This heat exchange can be quantified by measuring the change in enthalpy of the reaction.

Constant Temperature and Pressure:
In this specific scenario, the question states that the temperature and pressure are constant during the reaction. This means that the system is not experiencing any changes in these variables, allowing us to focus on the heat exchange aspect.

Entropy, Free Energy, and Internal Energy:
Entropy is a measure of the randomness or disorder of a system. It is not directly related to heat exchange at constant temperature and pressure.

Free energy, also known as Gibbs free energy, is a thermodynamic potential that combines the enthalpy and entropy of a system. It is used to determine the spontaneity of a reaction and is not directly related to heat exchange at constant temperature and pressure.

Internal energy is the sum of the kinetic and potential energies of the particles within a system. While it is related to heat exchange, it does not specifically refer to heat exchanged in a chemical reaction at constant temperature and pressure.

Conclusion:
Based on the given information and the definitions of the different terms, the correct answer is option B, enthalpy. Enthalpy specifically refers to the heat exchanged in a chemical reaction at constant temperature and pressure.