What percentage of air is oxygen by volume?

The composition of Earth's atmosphere reveals that approximately 21% of air consists of oxygen by volume. This percentage is significant because oxygen plays a vital role in supporting life through respiration for humans and other aerobic organisms. The remaining components of air include nitrogen, which makes up about 78%, along with trace gases such as argon, carbon dioxide, and others. Understanding this composition is fundamental in fields such as environmental science, biology, and engineering, especially in contexts like combustion processes and atmospheric studies. The choice of 21% accurately reflects the scientific consensus on atmospheric composition, making it the correct answer for the question.