Assertion : Heavy water is used as moderator in nuclear reactor.
Reason : Water cool down the fast neutron.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Heavy water is used as a moderator in nuclear reactors."
- Heavy water (D₂O) is indeed used in some nuclear reactors as a moderator. Its role is to slow down fast neutrons produced during fission reactions, making them more likely to cause further fission when they collide with fissile material (like Uranium-235).

2. **Understanding the Reason**:
- The reason given is "Water cools down the fast neutron."
- This statement is misleading. While water (including heavy water) does interact with fast neutrons, the term "cool down" is not accurate in this context. Instead, heavy water slows down fast neutrons through elastic collisions, which is a different process than cooling.

3. **Analyzing the Relationship**:
- The assertion is true because heavy water is indeed used as a moderator in nuclear reactors.
- The reason is false because it incorrectly describes the process of neutron moderation. Neutrons are slowed down, not cooled down.

4. **Conclusion**:
- Based on the analysis, we conclude that the assertion is true, but the reason is false. Therefore, the correct answer is that the assertion is true, and the reason is false.

### Final Answer:
- Assertion: True
- Reason: False
- Correct Option: Assertion is true, Reason is false.