Name any one material used as a moderator in a nuclear reactor.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Role of a Moderator:**
- A moderator in a nuclear reactor is a substance that slows down the fast neutrons produced during fission reactions. Slower neutrons are more likely to induce further fission in the fuel.

2. **Identifying Common Moderators:**
- There are several materials that can be used as moderators. The most common ones include:
- Ordinary (light) water (H2O)
...