Option 1 states that heavy water is extensively used as a moderator in nuclear reactors. This statement is correct. Heavy water, which contains a higher concentration of deuterium (a heavy isotope of hydrogen), is used as a moderator to slow down the fast neutrons produced in nuclear reactions.

Option 2 states that heavy water cannot be used in an exchange reaction to study reaction mechanism. This statement is incorrect. Heavy water can be used in exchange reactions, where deuterium from heavy water can replace normal hydrogen atoms in molecules, allowing scientists to study reaction mechanisms.

Option 3 states that the viscosity of heavy water is relatively smaller than that of ordinary water. This statement is incorrect. The viscosity of heavy water is actually higher than that of ordinary water. This is due to the larger size and higher mass of the deuterium atom in heavy water, which leads to stronger intermolecular forces and increased viscosity.

Option 4 states that the dielectric constant of heavy water is smaller than that of ordinary water. This statement is correct. The dielectric constant is a measure of a material`s ability to store electrical energy in an electric field. Heavy water has a smaller dielectric constant compared to ordinary water.

In conclusion, the correct statements are option 1 and option