Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Hydrogen is the most abundant element in the Universe, but it is not the most abundant gas in the troposphere.
Reason R: Hydrogen is the lightest element.
In the light of the above statements, choose the correct answer from the options given below:

Assertion A states that hydrogen is the most abundant element in the Universe, but it is not the most abundant gas in the troposphere. This is true because while hydrogen constitutes about 75% of the elemental mass in the Universe (mostly in stars), the most abundant gas in Earth's troposphere is nitrogen ($$N_2$$, about 78%), followed by oxygen ($$O_2$$, about 21%). Hydrogen gas is present in only trace amounts in the troposphere.

Reason R states that hydrogen is the lightest element. This is true, as hydrogen has an atomic mass of approximately 1 u, making it the lightest of all elements.

The reason R correctly explains assertion A. Because hydrogen is the lightest element, hydrogen molecules have the highest average molecular speed at any given temperature. At Earth's temperatures, a significant fraction of hydrogen molecules can exceed the escape velocity of Earth's gravitational field. Over geological timescales, most of the free hydrogen has escaped from the troposphere into space. This is why, despite being the most abundant element in the Universe, hydrogen is not the most abundant gas in the troposphere.

Therefore, the correct answer is Option (2): Both A and R are true and R is the correct explanation of A.