Approximately what percentage of air is made up of oxygen?

The correct answer is that approximately 21% of the Earth's atmosphere is made up of oxygen. This percentage is standard and widely accepted in various scientific resources, including geography, biology, and environmental science.

Understanding the composition of air is crucial for fields like building maintenance, where proper ventilation and indoor air quality are essential. Oxygen plays a vital role in combustion processes, which are commonly found in boilers, heating systems, and other equipment. Thus, having a clear grasp of these proportions can aid in ensuring that systems operate efficiently and safely.

The other percentages presented in the choices misrepresent the actual composition of oxygen in the atmosphere. This knowledge is fundamental not only for scientific clarity but also for practical applications in maintenance and safety protocols.