ANS:D - Heavy water & berrelium.

The correct set of materials used as a moderator in a nuclear reactor is: Heavy water & beryllium. Moderators are substances used in nuclear reactors to slow down neutrons, making them more likely to react with the fissile material (such as uranium-235 or plutonium-239) and sustain a nuclear chain reaction. Heavy water (deuterium oxide, D2O) and beryllium are both effective moderators. Heavy water is a form of water where the hydrogen atoms are replaced by deuterium, an isotope of hydrogen with an additional neutron in its nucleus. Beryllium is a metal that can also effectively slow down neutrons. So, the correct option is: Heavy water & beryllium.