Answer:
D. Water
Explanation:
Isotopes -
For some elements , there are various forms possible , which have different number of neutron and same number of protons , are referred to as the isotopes .
The three isotopes possible for the element hydrogen are -
Protium , Deuterium and Tritium .
Where ,
Deuterium is very rarely found on Earth , but can easily be prepared from water , which is known as deuterated water , which is helpful for isotopic labelling.