The correct answer is: A. Water vapor
Water vapor is a variable gas in the earth’s atmosphere because its concentration varies greatly depending on the temperature and humidity of the air. It is the most abundant greenhouse gas in the atmosphere, and it plays a significant role in regulating the Earth’s climate.
Nitrogen is the most abundant gas in the atmosphere, making up about 78% of the total volume. It is a non-reactive gas that does not play a significant role in the Earth’s climate.
Oxygen is the second most abundant gas in the atmosphere, making up about 21% of the total volume. It is a reactive gas that is essential for life.
Argon is the third most abundant gas in the atmosphere, making up about 0.9% of the total volume. It is a non-reactive gas that does not play a significant role in the Earth’s climate.