Which of the following is NOT a greenhouse gas?

Oxygen is not considered a greenhouse gas. Greenhouse gases are substances that can absorb and emit infrared radiation, leading to the greenhouse effect, which contributes to the warming of the Earth. These gases include water vapor, carbon dioxide, and methane, as they have molecular structures that allow them to trap heat in the atmosphere.

In contrast, oxygen does not have this property. Although oxygen is essential for life and plays a vital role in many chemical processes, it does not significantly contribute to the greenhouse effect because it cannot absorb infrared radiation to a notable extent. This distinction highlights the importance of understanding the specific roles different gases play in climate change and atmospheric science.