Which of the following is not considered a greenhouse gas?

Oxygen is not considered a greenhouse gas. Greenhouse gases are those that can trap heat in the Earth's atmosphere and contribute to the greenhouse effect. Carbon dioxide, methane, and ozone all have properties that allow them to absorb and emit infrared radiation, which contributes to the warming of the atmosphere.

Carbon dioxide and methane are well-known greenhouse gases due to their significant roles in human activities, particularly in relation to fossil fuel burning and agriculture. Ozone, while it occurs naturally in the atmosphere and has protective functions in the stratosphere, can also act as a greenhouse gas in the lower atmosphere where it is a product of pollution. On the other hand, oxygen is essential for life and organic processes but does not have the capacity to trap heat in the atmosphere, thus it does not contribute to climate change or the greenhouse effect.