Explanation

The function of a moderator in a nuclear reactor is to slow down neutrons so that they are more likely to cause further fission reactions. Among the options provided, C. Graphite is a good moderator. Graphite is commonly used in certain types of nuclear reactors, such as graphite-moderated reactors, because it effectively slows down neutrons due to its low atomic mass and structure. Helium, Cadmium, and Ordinary Water are not typically used as moderators in nuclear reactors. Helium has a low atomic mass but is used as a coolant in some reactors, Cadmium is often used as a neutron absorber in control rods, and Ordinary Water (H2O) is used as both a coolant and a moderator in pressurized water reactors, but it’s not as efficient a moderator as graphite.