Concept:A moderator is a material used to slow down fast neutrons in a nuclear reactor.
Explanation:Fast neutrons are produced during nuclear fission.
These neutrons must be slowed down so they can cause further fission reactions.
The substance that does this is called a moderator.
Graphite is a common moderator because it slows down neutrons effectively without absorbing too many.
Boron absorbs neutrons, so it is used in control rods, not as a moderator.
Copper is not suitable for slowing neutrons, and uranium is the nuclear fuel.
Therefore, among the given options, graphite is the correct material.
Answer:Graphite