The correct answer is option 4: Moderator. Heavy water, also known as deuterium oxide (D2O), is used as a moderator in nuclear reactors.

In a nuclear reactor, the role of a moderator is to slow down the fast-moving neutrons released during the fission of uranium-235 or plutonium-239. Slowing down these neutrons increases the chances of them being captured by other fissile atoms, leading to more sustained fission reactions.

Heavy water is an excellent moderator because it contains a higher proportion of deuterium, a heavy isotope of hydrogen. Heavy water slows down neutrons more effectively than regular water (which contains a higher proportion of light hydrogen) or other materials commonly used as moderators, such as graphite.

While heavy water can also be used as a solvent or catalyst in certain chemical reactions, its primary use in nuclear reactors is as a moderator.