The correct answer is: B. Graphite
A moderator is a material used in a nuclear reactor to slow down (moderate) the neutrons produced by the fission of uranium-235 or plutonium-239. This is necessary to increase the probability that these neutrons will cause further fissions, thus sustaining the chain reaction.
Graphite is a good moderator because it is a good conductor of heat, which helps to keep the reactor core at a constant temperature. It is also relatively inexpensive and easy to work with.
Thorium is a naturally occurring radioactive metal that can be used as a fuel in nuclear reactors. However, it is not as efficient as uranium-235 or plutonium-239, and it produces more radioactive waste.
Radium is a highly radioactive metal that is not used in nuclear reactors. It is used in some medical treatments, but it is too dangerous to be used in a reactor.
Ordinary water is not a good moderator because it absorbs neutrons. This can lead to a decrease in the number of neutrons available to cause fission, and can eventually cause the reactor to shut down.