Which substance is used as moderator to slow down fast neutrons in a nuclear reactor?

The correct answer is Option C — Heavy water and Graphite. A moderator is used for this purpose. Heavy water (D₂O) and graphite (carbon) are the most commonly used moderators. They slow neutrons without absorbing them significantly. Option A is incorrect: Cadmium is used as a control rod material, not a moderator. Cadmium absorbs neutrons very efficiently, and control rods made of cadmium are inserted into the reactor to reduce the number of neutrons and control the reaction rate. Option B is incorrect: Lead is used as radiation shielding material in nuclear facilities because it is dense and absorbs gamma rays. It is not used as a moderator because it does not effectively slow neutrons. Option D is incorrect: Uranium is the fuel in the reactor, not the moderator. U-235 undergoes fission when it absorbs a slow neutron; it does not slow down neutrons. Regular water (H₂O) can also act as a moderator in light water reactors.