Answer: D. Cadmium
Explanation: Control Rods are the rods are which are used in the nuclear reactor.
They are used to control nuclear fission rate of uranium and plutonium.These elements help in absorbing neutrons within themselves.
Thus out of plutonium, liquid sodium , carbon, and cadmium, Cadmium is used as the control rod.
If these control rods are removed, then there will be production of more heat due to the presence of neutrons .