What is the primary composition of Earth's atmosphere?

The primary composition of Earth's atmosphere is accurately described as consisting of nitrogen, oxygen, and argon, specifically with nitrogen making up approximately 78% of the atmosphere, oxygen about 21%, and argon around 1%. This combination forms the majority of the atmosphere, which is crucial for supporting life on Earth. Nitrogen is a relatively inert gas that plays a significant role in biological processes, while oxygen is essential for respiration in animals and combustion processes.

Knowing these percentages is vital for understanding various atmospheric and environmental sciences, including weather patterns, climate changes, and how gases interact within the atmosphere. The composition reflects a stable state that has been maintained for thousands of years, ensuring a balance necessary for life.

The other provided options do not accurately reflect the true composition of Earth's atmosphere. They either overstate the presence of certain gases or misrepresent the proportions entirely. For instance, carbon dioxide is indeed present in small amounts within the atmosphere but does not constitute 90% of it, and hydrogen is not a significant component in the atmosphere compared to nitrogen and oxygen. This emphasizes the uniqueness of Earth's atmospheric composition and its importance to ecological and meteorological studies.