Explanation:
Entropy is a measure of the disorder or randomness in a system. The Second Law of Thermodynamics states that the total entropy of an isolated system always increases over time. Therefore, if the entropy of the universe decreases, it would violate this fundamental law and be considered an impossible process.

Understanding the Options:
Let's analyze the other options to understand why they are not correct in this scenario:

a) Ideal Process: An ideal process is one that occurs without any losses or inefficiencies. It is characterized by reversible changes, and the entropy of the system remains constant. However, the question states that the entropy of the universe decreases, which is not possible in an ideal process.

b) Reversible Process: A reversible process is one that can be reversed by an infinitesimal change in the system's conditions. In a reversible process, the system and its surroundings can be brought back to their initial state without any net entropy change. Since the question states that the entropy of the universe decreases, it contradicts the characteristics of a reversible process.

c) Irreversible Process: An irreversible process is one that occurs with losses or irreversibilities. It is characterized by an increase in the entropy of the system and the surroundings. The entropy of the universe always increases in an irreversible process, so option (c) is also incorrect.

Conclusion:
Based on the Second Law of Thermodynamics, the entropy of the universe always increases or remains constant. If the entropy of the universe were to decrease, it would violate this fundamental law and be considered an impossible process. Therefore, option (d) is the correct answer.