Heavy water implies

Heavy water, chemically known as deuterium oxide (D2O), is water in which the standard hydrogen atoms (protium) are replaced by deuterium, a stable isotope of hydrogen [1]. Deuterium contains one proton and one neutron, making it approximately twice as heavy as normal hydrogen [1]. Consequently, heavy water is about 10% denser than ordinary water. It is primarily used as a neutron moderator and coolant in nuclear reactors, such as Pressurized Heavy Water Reactors (PHWR), because it absorbs fewer neutrons than light water, facilitating sustained nuclear fission [1]. While option 2 describes 'hard water' (containing calcium and magnesium salts) and option 4 describes a physical property of water at 4°C, 'heavy water' specifically refers to the isotopic composition involving deuterated water.

Sources

1. [1] https://hwb.gov.in/nuclear-applications