You may have noticed that when water boils, you can see bubbles that rise to the surface of the water.

You know it can't be air; there was no air to begin with.

You know the water didn't decompose so ii can't be right.
Same reasoning for why iii can't be right.
No reason to think CO2 can be right.
So what's left. Wouldn't you think water vapor would be reasonable?

iv. water gas

The answer is i. air.

The bubbles that rise to the surface of the boiling water contain air.

To understand this, we need to know that when water boils, it undergoes a phase change from a liquid to a gas. As the temperature of the water increases, the water molecules gain enough energy to escape from the liquid phase and turn into water vapor, which is a gas.

The bubbles we see are formed when the water vapor tries to escape from the liquid and rises to the surface. These bubbles are primarily composed of air, which is a mixture of various gases, including nitrogen, oxygen, and small amounts of other gases.

Therefore, the correct answer is (i) air.