Which gas constitutes the largest percentage of normal air?

In the composition of normal air, nitrogen is the most abundant gas, making up about 78% of the atmosphere by volume. This prevalence is significant because nitrogen plays a crucial role in the Earth's atmosphere, aiding in various biological and chemical processes while being relatively inert and not directly reactive with most other gases at typical atmospheric conditions.

Oxygen, while vital for the respiration of living organisms and constituting approximately 21% of the atmosphere, is still far less prevalent than nitrogen. Carbon dioxide and water vapor are present in much smaller amounts, typically around 0.04% and variable percentages (up to about 4% for water vapor, depending on the climate), respectively. Thus, nitrogen's dominance in the atmosphere makes it the correct choice for this question regarding the gas that constitutes the largest percentage of normal air.