At constant volume and temperature, the change in internal energy is equal to

 At constant volume, the heat of reaction is equal to the change in the internal energy of the system. At constant pressure, the heat of reaction is equal to the enthalpy change of the system. At constant volume, no PV work can be done, and the change in the internal energy of the system is equal to the amount of heat transferred from the system to the surroundings or vice versa.

Hence the correct answer is (B) amount of heat exchanged.