Assertion: An adiabatic process in which no work is performed is an example of a constant energy process.
Reason: The change in enthalpy attending an isothermal change in state of an ideal gas must be equal to the change in energy.
(A) Both reason and assertion are correct and reason is the correct explanation for assertion.
(B) Both reason and assertion are correct but reason is not the correct explanation for assertion.
(C) Assertion is correct but reason is incorrect.
(D) Assertion is incorrect but reason is correct.

Hint : In thermodynamics, an adiabatic process can be defined as a thermodynamic process which takes place without transferring heat or mass between the system and its surroundings but it transfers energy to its surroundings only as work. In this process, we can notice gas compression and heat is generated.

Complete Step By Step Answer:
A thermodynamic process in which the temperature of a system is constant is called isothermal process. Thermal equilibrium is maintained as the transfer of heat into or out of the system takes place very slowly. Heat is not transferred between the system and its surroundings in an adiabatic process. For a closed system, we can write the first law of thermodynamics as: $\Delta U = Q - W$
For an adiabatic process: $Q = 0$
If the walls of the system are adiabatic, then the change in internal energy is equal to the work done. In case of an isothermal change in state, there will be no change in energy.
Therefore, option C is the correct answer.

Additional Information:
The state of a thermodynamic system is expressed using various parameters like temperature, pressure, volume and internal energy. The state of a given system can be changed by different processes like isothermal, isobaric, isochoric and adiabatic processes. The temperature is kept constant in an isothermal process. The temperature is kept constant in an isothermal process. The pressure is kept constant in an isobaric process. The volume is kept constant in an isochoric process. There is no heat transfer in adiabatic processes.

Note :
If no work is performed, then an adiabatic process is an example of a constant energy process. In an isothermal process, the change in temperature is zero and the temperature is maintained constant by the system.