B

Step 1: Understanding Moderators
In nuclear reactors, a moderator is a substance that slows down neutrons, making them more likely to induce fission in fissile material like uranium-235 or plutonium-239.

Step 2: Evaluating the Options
- Option A: Uranium - Uranium itself is primarily the fuel in most reactors and does not serve as a moderator.
- Option B: Heavy water - Heavy water (D₂O) is an effective neutron moderator and is used in several types of reactors, particularly CANDU reactors.
- Option C: Cadmium - Cadmium is often used as a neutron absorber, not as a moderator, since it captures neutrons rather than slowing them down.
- Option D: Plutonium - Similar to uranium, plutonium is used as fuel and does not function as a moderator.

Step 3: Conclusion
Based on the definitions and roles of each substance in the context of nuclear reactors, the most appropriate answer is Heavy water (Option B) as it is indeed used as a moderator in nuclear reactors.