Which gas makes up approximately 21% of the Earth's atmosphere?

Oxygen is the correct answer because it constitutes roughly 21% of the Earth's atmosphere, making it one of the most crucial gases for supporting life as we know it. This gas is vital for the survival of most living organisms on the planet, as it is required for the process of respiration. During this process, animals and humans take in oxygen and exhale carbon dioxide, contributing to the balance of gases in the atmosphere.

The presence of oxygen at this concentration is also important for combustion and various chemical reactions, influencing both environmental conditions and climate. Its relatively stable proportion in the atmosphere is a result of both biological contributions, such as photosynthesis carried out by plants, and geological processes.

In contrast, argon is a noble gas that makes up about 0.93% of the atmosphere, carbon dioxide is present in much smaller amounts (around 0.04%), and nitrogen accounts for approximately 78%. Understanding these compositions is key to grasping atmospheric science and the fundamental principles that govern ecological balance.