The substance used as a moderator in the reactor of a nuclear power plant is:
A) boron
B) iron
C) graphite
D) mercury

Hint: The material that slows down the neutrons in the chain reaction is called a moderator. These substances slow the neutron and provide more time to initiate the chain reaction in the nuclear fission.

Complete step by step answer:
Nuclear reactors are used to produce electricity through nuclear energy. Nuclear energy generates in the nuclear reaction. The nuclear reaction may be of two types; one is nuclear fission and the second is nuclear fusion.
Nuclear fission is the reaction in which the heavy nucleus splits into different small nuclei. A large amount of energy produced in nuclear fission.
Nuclear fusion is the reaction in which different light nuclei form a heavy nucleus. This reaction requires some energy and high temperature for the initiation of the reaction. A large amount of energy also produces nuclear fusion.
Nuclear fission is used in the nuclear reactor for electricity production. The three neutrons generate in every nuclear fission. These neutrons further initiate another nuclear fission reaction. The neutron leaves the surface very fast, so moderator substances are used to slow down the neutron for a controlled chain reaction.

Therefore, the substance used as a moderator in the reactor of a nuclear power plant is graphite and the option (C) is correct.

Note:
- The uncontrolled chain reaction damages the reactor, so only a controlled chain reaction is used for power generation.
- The chain reaction may be controlled or uncontrolled. The control rods are used to control the number of the neutrons and coolant is used to control the heat in the reactor.