What is the approximate composition of air by percentage?

The composition of air is approximately 21% oxygen and 79% nitrogen. This ratio is essential for various biological and chemical processes on Earth. Oxygen is crucial for respiration in most living organisms, while nitrogen serves as an inert gas that dilutes oxygen and prevents rapid combustion. The percentages remain consistent under standard atmospheric conditions, making this information fundamental for understanding respiratory processes and combustion in both natural and industrial contexts.

Understanding this composition helps in many practical applications, such as maintaining atmospheres in controlled environments, understanding air quality standards, and optimizing combustion processes in engines and boilers. The other choices present incorrect ratios that do not reflect the actual composition of air, which is consistent across diverse environments on Earth.