Correct Answer: (e) both heavy water and ordinary water.
Solution:Heavy water(D2O), also called deuterium oxide, contains a larger than normal amount of the hydrogen isotope deuterium. It is used as moderator in the reactor to slow down the neutrons produced in fission, so that the chain reaction can sustained. Many reactors also use Light water, normal water (H2O), Graphite of Beryllium oxide as moderator. In India, mainly heavy water is used as moderator in nuclear reactors.