Which of the following is NOT a greenhouse gas?

1. A

   Argon

   Option (a) claims that Argon is not a greenhouse gas. This is factually correct because noble gases like argon do not absorb infrared radiation effectively and thus play no direct role in the greenhouse effect, making this the correct choice.

2. B

   Chlorofluorocarbon

   Option (b) claims that Chlorofluorocarbon is not a greenhouse gas. This statement is false because CFCs are potent synthetic greenhouse gases that trap heat in the atmosphere and contribute significantly to global warming.

3. C

   Methane

   Option (c) claims that Methane is not a greenhouse gas. This statement is incorrect because methane is a major greenhouse gas emitted from agricultural activities, fossil fuel production, and natural wetlands, with a high global warming potential.

4. D

   Carbon dioxide

   Option (d) claims that Carbon dioxide is not a greenhouse gas. This statement is wrong as carbon dioxide is the most prominent anthropogenic greenhouse gas driving climate change through fossil fuel combustion and deforestation.

Summary. Official key is (a) Argon is not a greenhouse gas. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases responsible for trapping heat in Earth's atmosphere. Argon is an inert noble gas comprising about 0.93 percent of the volume of dry air, but it possesses no dipole moment and absorbs virtually no thermal radiation. Chlorofluorocarbons, methane, and carbon dioxide are all well-known heat-trapping gases monitored globally. Therefore, option (a) is the only correct choice among the given alternatives.