The correct answer to the question is: C) They control the rate of reaction.
Step-by-step explanation:
The control rod plays a crucial role in nuclear reaction. The main function of the control rod is to control the rate of reaction.
The nuclear chain reaction becomes uncontrolled when number of neutrons are not controlled. It is so because each decay of radioactive uranium or plutonium produces three extra neutrons which again decay the radioactive sample at a faster rate.
The control rods are made up of boron,indium and cadmium like substances. They absorb the thermal neutrons generated in each decay of radioactive specimen. In this way, they control the rate of reaction.