The correct answer is option 4, Oxygen. Oxygen is a gas that supports combustion, meaning it is necessary for a substance to burn. When a substance undergoes combustion, it combines with oxygen to release heat and light energy. Oxygen acts as an oxidizing agent, enabling the substance to undergo oxidation and produce combustion. This is why it is often referred to as the "fuel for fire."

Option 1, Hydrogen, is a combustible gas that can burn in the presence of oxygen, but it itself is not a supporter of combustion.

Option 2, Nitrogen, is an inert gas that does not support combustion. It does not react with other substances and is often used as a fire extinguisher to remove oxygen from a fire.

Option 3, Carbon dioxide, is also an inert gas that does not support combustion. It is produced as a byproduct of combustion and can actually smother a fire by displacing oxygen.

In conclusion, Oxygen is the gas that supports combustion, while Hydrogen, Nitrogen, and Carbon dioxide do not.