What percentage of air is made up of oxygen?

The correct answer is 20.9 percent, which represents the approximate percentage of oxygen in the Earth’s atmosphere. Understanding this value is important for various applications, including those related to safety and operation within environments where gases are present, such as in natural gas systems.

The atmosphere is primarily composed of nitrogen, which makes up around 78 percent, while oxygen constitutes about 20.9 percent. This ratio influences combustion processes, respiratory functions, and is a fundamental aspect of many scientific and industrial fields.

The other options, while close, either underestimate or overestimate the actual percentage of oxygen in air, highlighting the importance of knowing precise atmospheric compositions for effective training and practice in fields such as natural gas certification.