Final answer:
Nitrogen (option A) is the gas that comprises the largest share of Earth's atmosphere by volume, making up roughly 78% of it.
Step-by-step explanation:
The gas that makes up the majority of Earth's atmosphere by volume is nitrogen. Nitrogen constitutes about 78% of the Earth's atmosphere, and it serves crucial roles, such as diluting oxygen to prevent rapid burning and being used by living things to make proteins. Following nitrogen, the most abundant gases are oxygen, which makes up approximately 21%, and then argon, at about 0.9%. Carbon dioxide is a minor component of the atmosphere, contributing to only about 0.04% of its volume.