Which of the following substances does not release $O_2$ and removes water vapor?

The natural relative abundance of isotopes of hydrogen is:

The basic principle of the hydrogen economy is the transportation and storage of energy in the form of liquid or gaseous hydrogen. Which property of hydrogen may be useful for this purpose? Support your answer with a chemical equation if required.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Physical properties of isotopes of hydrogen are different.
Reason $R$: Mass difference between isotopes of hydrogen is very large.
In the light of the above statements,choose the correct answer from the options given below:

Water gas is a mixture of .........

Which hydride among the following is less stable?