What is the most common gas in the Earth's atmosphere?

The most common gas in the Earth's atmosphere is nitrogen, which constitutes about 78% of the atmosphere by volume. This high concentration is significant because nitrogen is a relatively inert gas, meaning it does not readily react with other substances under normal conditions. Its stability plays a crucial role in maintaining the overall composition of the atmosphere.

While oxygen is vital for most life forms and makes up about 21% of the atmosphere, it is not as abundant as nitrogen. Carbon dioxide, although important for processes like photosynthesis and for regulating Earth's temperature, exists in much smaller amounts, typically around 0.04%. Hydrogen, while it is the lightest and most abundant element in the universe, constitutes only trace amounts in Earth's atmosphere, primarily due to its low density and tendency to escape into space.

Thus, the dominance of nitrogen in the composition of the atmosphere is what makes it the correct answer to this question.