Which one of the following is NOT a greenhouse gas found naturally in the atmosphere?

The answer is A) Nitrogen oxide, though the wording needs care. The question is testing the difference between natural greenhouse gases and the pollutant oxides that come mainly from human activity.

• Carbon dioxide (B) is a natural greenhouse gas. It enters the atmosphere through respiration, volcanic activity and ocean exchange, and it is the main long-lived gas in the Earth's natural greenhouse effect.
• Methane (C) is also natural. It comes from wetlands, termites and enteric fermentation, and it is far more potent per molecule than CO2 over a 20-year horizon.
• Ozone (D) is natural too. Stratospheric ozone shields UV radiation, and tropospheric ozone acts as a greenhouse gas.
• Nitrogen oxide (A) is the option that fits the question's intent. Oxides of nitrogen (NOx) are mainly combustion pollutants from vehicles and power plants, and they are not listed among the standard natural greenhouse gases. Nitrous oxide (N2O) is a natural greenhouse gas, so the distinction matters, and examiners often blur it.

The core concept is the natural versus anthropogenic classification of greenhouse gases, which links to Environment and Ecology and to Mains answers on climate change mitigation, where sources of emissions must be identified.

Trap: candidates see "nitrous oxide" as a greenhouse gas and then wrongly reject the generic term "nitrogen oxide" as well, or they pick ozone because it is often called a pollutant.