The correct answer is (a) Carbon dioxide.
Carbon dioxide is the most important greenhouse gas because it is the most abundant. It is also the most long-lived, meaning it stays in the atmosphere for a long time. Other greenhouse gases, such as methane and nitrous oxide, are also important, but they are not as abundant as carbon dioxide.
Water vapor is also a greenhouse gas, but it is not as important as carbon dioxide because it is not as long-lived. Water vapor is also constantly being exchanged between the atmosphere and the Earth’s surface, so its concentration in the atmosphere is not as stable as carbon dioxide.
Ozone is a greenhouse gas, but it is not as important as carbon dioxide because it is not as abundant. Ozone is also constantly being created and destroyed in the atmosphere, so its concentration is not as stable as carbon dioxide.
In conclusion, carbon dioxide is the most important greenhouse gas because it is the most abundant, the most long-lived, and the most stable.