C. Steam
Explanation - In nuclear reactor, chain of reaction takes place that involves the nuclear substances to react. During the reaction, large amount of heat is produced and to remove this heat, coolant is used. A Coolant can either be liquid or a gas.
The coolant keeps the fuel cool or prevents it from overheating. The coolant should have high specific heat, good conductivity, better chemical stability, better pumping characteristics and low area for neutron absorption.
