What substances are used as moderators in a nuclear reactor?
a. carbon and water
b. liquid sodium and water
c. plutonium and neptunium
d. cadmium or other metals
ansewer is a. carbon & water

 A. Carbon and Water

Explanation:  Moderators are the substances which are used to slow down the fast moving neutrons from the thermal fission in the nuclear reactor.

They usually help to maintain the chain reaction which are propagated through the interaction of the heavy nucleus of the uranium and the neutrons.

Other than carbon  and light water(H2O), sometimes heavy water(D2O) is also used as moderator.

Thus graphite (allotrope of carbon) and light water is used as moderator in nuclear reactor.