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

Oxygen is the gas that constitutes approximately 21% of the Earth's atmosphere. This percentage is significant as oxygen is essential for the survival of most living organisms on the planet, allowing for processes such as respiration in animals and photosynthesis in plants. The presence of oxygen in such a proportion is crucial for maintaining life and supporting various biological and chemical processes.

In contrast, nitrogen makes up about 78% of the atmosphere, making it the most abundant gas, but it is not the one that comprises 21%. Carbon dioxide is present only in trace amounts, around 0.04%, and plays an important role in regulating Earth's temperature and supporting plant life, but it is not what the question specifies. Hydrogen, while it is an essential element, is found in very small amounts in the atmosphere compared to other gases. Hence, oxygen stands out as the correct answer due to its critical role and specific percentage in the Earth's atmosphere.