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

Moderators are used in nuclear reactors to slow down the speed of neutrons. By slowing the neutrons down the probability of a neutron interacting with Uranium235 nuclei is greatly increased thereby maintaining the chain reaction. The neutrons produced by nuclear fission are very fast. At that speed the probability of the neutron causing a fission with another uranium or plutonium becomes low. Neutrons become slow by colliding with a moderator and thus the probability of further fission increases. Solid graphite, heavy water are some of the moderators used. Hence, option D is correct.