What is enthalpy?

A. Enthalpy is the kinetic energy of a system.
B. Enthalpy is the heat involved in a reaction.
C. Enthalpy is the temperature of a reaction.
D. Enthalpy is the mass involved in a reaction.
I think the answer is a or b

You're right. It's a or b but it is not kinetic energy.

http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/State_Functions/Enthalpy

The correct answer is B. Enthalpy is the heat involved in a reaction. To determine the answer, you can recall the definition of enthalpy or refer to a reliable source such as a chemistry textbook or trusted website. Enthalpy is a thermodynamic property of a system, which refers to the total heat content of that system. It is represented by the symbol H and is often measured in units such as joules (J) or calories (cal). Enthalpy takes into account both the internal energy of a system and the work done by or on the system. Additionally, enthalpy changes are commonly used to describe the heat transferred in chemical reactions.