The substance used as moderator and coolant both, in nuclear reactors is

Correct answer: (b) Heavy water

Explanation

Heavy water (D₂O), or deuterium oxide, acts as both moderator and coolant in many nuclear reactors.

As a moderator it slows down fast neutrons without absorbing them, sustaining the chain reaction.

Ordinary water absorbs neutrons more readily, while liquid ammonia and liquid hydrogen are not used for this dual role.