Which of the following is NOT a greenhouse gas?

Oxygen is not considered a greenhouse gas because it does not significantly contribute to the greenhouse effect. Greenhouse gases are substances in the atmosphere that can absorb and emit infrared radiation, leading to the warming of the Earth's surface. Carbon dioxide, methane, and nitrous oxide all possess molecular structures that allow them to trap heat effectively, contributing to the greenhouse effect and helping to regulate the Earth's temperature.

In contrast, oxygen, while being a vital component of the atmosphere and critical for life on Earth, does not have the capacity to absorb infrared radiation like the other gases. Therefore, it does not play a role in the greenhouse effect, distinguishing it clearly from the greenhouse gases listed.