Which of the following is used as ‘a moderator in nuclear reactor?

The moderator of a nuclear reactor is a substance that slows neutrons down. In traditional nuclear reactors, the moderator is the same thing as the coolant: it’s water! When fast neutrons strike the hydrogen atoms in H₂O, they slow down a lot (like a billiard ball striking another). There are other good moderators like graphite, beryllium, and more.

Answer : Graphite