In a nuclear reactor, what is the function of:
The moderator,

Explanation

The moderator is a material which slows down the speed of the neutrons. In nuclear chain reactions, generally heavy water `(D_2O)` is used as moderator but some reactors also uses graphite. By controlling the amount of moderator material in the reactor you can maintain the reaction in a controlled state, i.e. the nuclear reaction is sustained, producing heat, but not leading to an uncontrolled explosion.