Assertion : All spontaneous processes are thermodynamically irreversible.
Reason : In a reversible isothermal process, there is no net change in entropy.

Options

A. If both Assertion and Reason are CORRECT and Reason is the CORRECT explanation of the Assertion.
B. If both Assertion and Reason are CORRECT but Reason is not the CORRECT explanation of the Assertion.
C. If Assertion is CORRECT but Reason is INCORRECT.
D. If Assertion is INCORRECT but Reason is CORRECT.

Explanation

To analyze the given assertion and reason, let's break down the concepts step by step.

### Step 1: Understanding the Assertion
**Assertion:** All spontaneous processes are thermodynamically irreversible.

- A spontaneous process is one that occurs naturally without the need for external energy input. In thermodynamics, spontaneous processes are characterized by an increase in the total entropy of the system and its surroundings.
- The second law of thermodynamics states that the entropy of an isolated system always increases over time for spontaneous processes. This means that spontaneous processes cannot be reversed without external work being done, making them irreversible.

### Step 2: Understanding the Reason
**Reason:** In a reversible isothermal process, there is no net change in entropy.

- A reversible process is an idealized process that can be reversed without leaving any change in the system or surroundings. In a reversible isothermal process, the system is in thermal equilibrium with its surroundings, and any heat transfer occurs without a change in temperature.
- In such a process, the entropy change of the system is exactly balanced by the entropy change of the surroundings, leading to no net change in the total entropy of the universe. This is a characteristic of reversible processes.

### Step 3: Evaluating the Assertion and Reason
- The assertion is true because all spontaneous processes are indeed thermodynamically irreversible due to the increase in entropy.
- The reason is also true as it correctly describes a reversible isothermal process where there is no net change in entropy.

### Step 4: Conclusion
While both the assertion and the reason are true, the reason does not adequately explain the assertion. The assertion's explanation lies in the fact that spontaneous processes lead to an increase in entropy, which is not the case in reversible processes.

### Final Answer
- **Assertion:** True
- **Reason:** True
- **Conclusion:** Both the assertion and reason are true, but the reason is not the correct explanation for the assertion.