Heavy water ($D_2O$), also called deuterium oxide, water composed of deuterium, the hydrogen isotope with a mass double that of ordinary hydrogen, and oxygen. (Ordinary water has a composition represented by $H_2O$.) Thus, heavy water has a molecular weight of about 20 (the sum of twice the atomic weight of deuterium, which is 2, plus the atomic weight of oxygen, which is 16),
The weight of one mole of heavy water is 20gms.
Hence (b) is the correct answer.