What percentage of oxygen is found in air by volume?

The correct answer is 21%, which reflects the average concentration of oxygen in Earth's atmosphere. This percentage is derived from extensive atmospheric studies, indicating that oxygen constitutes approximately one-fifth of the air we breathe.

Understanding the composition of air is fundamental in fields such as engineering, environmental science, and health, as it influences combustion processes, respiratory health, and atmospheric chemistry. Other gases present in the atmosphere include nitrogen (around 78%) and trace amounts of other gases such as carbon dioxide, argon, and neon. This established ratio of gases, including the consistent level of oxygen, is critical for sustaining life and maintaining ecological balance.

Choosing 20% or higher percentages, such as 23% and 25%, misrepresents the data and reflects common misconceptions about air composition. The slight variance from 21% to 20% is not accurate enough, while options like 23% and 25% suggests a significantly higher oxygen concentration than what is physically present in the atmosphere.