Correct Answer: Moderator

Solution : The correct option is Moderator.

Graphite is used in nuclear reactors as a "moderator." A moderator is a substance that slows neutrons generated during nuclear fission processes. Slowing down neutrons increases their chance of interacting with fissile materials like uranium-235 or plutonium-239, boosting the possibility of subsequent nuclear reactions or maintaining a nuclear chain reaction. Graphite is a frequent moderator material in some types of nuclear reactors, such as graphite-moderated reactors.