Moderator is used in nuclear reactor for:
A. Slowing neutrons
B. Accelerating neutrons
C. Stopping neutrons
D. Heating neutrons

Hint: To continue the chain reaction in a nuclear reactor we have to control the speed of neutrons released from the fission reaction.

Complete step by step answer:
A moderator is a substance used to reduce the speed of neutrons. Therefore, it can sustain nuclear chain reaction in the reactor. It can increase the probability of collision between neutrons and fuel rods. Water, heavy water and graphite are the examples for moderators. Therefore, the correct option is A.

Additional information:
Moderators must have specifications for the purpose. They are,
Non-corrosiveness
High melting point for solids and low melting point for liquids
Chemical and radiation stability
High thermal stability
A nuclear reactor contains not only moderators but also fuel, control rods, coolant and containment.
Fuel: Uranium is the basic fuel used for the fission reaction. For this, pellets of uranium oxide are made into fuel rods and placed in the core of the reactor.
Control rods: These are made up of neutron absorbing materials like cadmium or hafnium. These can control the rate of production of neutrons. Thus, the chain reaction can be controlled.
Coolant: A fluid used to remove the excess heat from the system. It helps to transfer the heat from the system to outside.
Containment: It is the structure to protect the system from outside intrusion and to protect outside from the effects of radiation.

Note: Absorption of neutrons are done by the control rods. Moderators will try to reduce the speed of the neutrons. While neutrons get absorbed by control rods if the number of neutrons is more.