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

1. A

   Nitrogen oxide

   Nitrogen oxide (NOx as commonly meant here) is not listed among the naturally occurring greenhouse gases in this syllabus framing, unlike CO2, CH4 and O3. That is the official key.

2. B

   Carbon dioxide

   Carbon dioxide is the principal long-lived natural and anthropogenic greenhouse gas.

3. C

   Methane

   Methane is a potent natural GHG from wetlands, ruminants and termites, besides leaks.

4. D

   Ozone

   Ozone in the troposphere acts as a greenhouse gas; stratospheric ozone is a separate UV shield.

Summary. Official key is (a) nitrogen oxide. The standard natural GHG set in this item is carbon dioxide, methane and ozone (and, unspoken, water vapour). Do not confuse NOx with nitrous oxide (N2O), which is a GHG. Honour the stored letter (a).