Heavy Water is

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Answer: (3) deuterium oxide.
Heavy water is deuterium oxide.

Heavy water consists of heavy hydrogen or deuterium and oxygen. It is also known as hard water and chemical formula is D₂O.  Deuterium differs from the hydrogen usually found in water.  It’s density is 1.107 gm/cm³ and boiling point is 101.4^oC.
It makes up a small percentage (0.02%) of water naturally founded on Earth.