The net change in internal energy is zero. Hence the correct option is d.
The net change in internal energy is zero when a system undergoes a cyclic process, returning to its initial state. In this scenario, the system goes from state A to B by a reversible path, experiencing a certain internal energy change of 40 kJ/mol. Subsequently, it returns from state B to A by an irreversible path. In a cyclic process, the net change in internal energy is determined by the initial and final states. Since the system returns to its initial state, the overall internal energy change is zero.
The reversible and irreversible paths essentially cancel each other out, resulting in no net change in internal energy. This concept aligns with the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. In a cyclic process, these contributions sum up to zero. Hence the correct option is d.