The correct answer for this question is option 3, heavy water.

Option 1, ozone, is not used as a moderator in nuclear reactors. Ozone is a form of oxygen with three oxygen atoms and is typically used as a disinfectant or oxidizer.

Option 2, heavy hydrogen, is also known as deuterium. While heavy hydrogen is used in some nuclear reactions, it is not commonly used as a moderator in nuclear reactors.

Option 4, hydrogen peroxide, is not used as a moderator in nuclear reactors. Hydrogen peroxide is a chemical compound that is commonly used as a bleaching agent, disinfectant, or oxidizer.

Heavy water, or deuterium oxide (D2O), is the correct answer. It is a form of water where the hydrogen atoms are replaced with deuterium, a heavy isotope of hydrogen. Heavy water is used as a moderator in some types of nuclear reactors, such as pressurized heavy water reactors (PHWRs). The heavy water slows down the neutrons produced during nuclear fission, increasing the probability of further reactions and sustaining the chain reaction in the reactor.