Commonly used moderators are
water
heavy water `(D_2 O)`.
graphite
sodium chloride (NaCl).

Options

A. I,II and III
B. I and II
C. I,II and IV
D. All of these

Answer

### Step-by-Step Solution:

1. **Define a Moderator**:
A moderator is a material used in nuclear reactors to slow down fast neutrons, which increases the likelihood of these neutrons being captured by the fuel nuclei.

2. **Properties of a Moderator**:
- **Low Neutron Absorption Cross-Section**: This means that the material does not absorb neutrons effectively, allowing more neutrons to interact with the fuel.
- **High Thermal Conductivity**: This property ensures that the heat generated in the reactor can be efficiently transferred away from the reactor core.
- **High Heat Capacity**: This allows the moderator to absorb and retain heat without a significant increase in temperature.

3. **Commonly Used Moderators**:
- **Water**: Regular water (H₂O) is widely used due to its availability and effective moderation properties.
- **Heavy Water (D₂O)**: Heavy water is more effective than regular water because it has a lower neutron absorption cross-section.
- **Graphite**: Graphite is another effective moderator that slows down neutrons without absorbing them significantly.
- **Sodium Chloride (NaCl)**: Although mentioned, sodium chloride is not typically used as a moderator due to its inability to absorb heat effectively.

4. **Conclusion**:
The commonly used moderators in nuclear reactors are water, heavy water, and graphite. Sodium chloride, while listed, is not effective as a moderator.