What is the sum of neutrons and protons in the heaviest isotope of hydrogen?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Hydrogen is an environment-friendly fuel.
Reason $R:$ Atomic number of hydrogen is $1$ and it is a very light element.
In the light of the above statements,choose the correct answer from the options given below:

The property of hydrogen which distinguishes it from other alkali metals is

Which is the lightest gas?

Write the names of the isotopes of hydrogen. What is the mass ratio of these isotopes?

Which of the following explanations is best for not placing hydrogen in either the group of alkali metals or halogens?