Final answer:
The change in entropy is Q/T specifically for reversible processes, where the entropy of the universe remains unchanged.
Step-by-step explanation:
The change in entropy (ΔS) is given by the equation ΔS = Q/T for reversible processes. In such processes, all intermediate states are equilibrium states, and the process can be reversed at any moment without changing the entropy of the universe. For irreversible processes, which include all real engines, entropy increases because dissipative elements like friction cause heat to be lost to the surroundings, which is not the case in a perfectly reversible process. Therefore, the correct answer to the student's question is A) reversible processes.
Reversible processes ensure no entropy change in the universe, while irreversible processes always lead to an entropy increase. This is a restatement of the second law of thermodynamics, which indicates that entropy remains constant if a process is reversible, and increases if a process is irreversible.