Which greenhouse gas does not exist naturally?

Chlorofluorocarbons (CFCs) are the greenhouse gases that do not exist naturally. These compounds are entirely synthetic, created by humans for use in refrigeration, air conditioning, and aerosol propellants, among other applications. Their development in the early 20th century led to significant environmental concerns, particularly their role in ozone depletion and as potent greenhouse gases that contribute to climate change.

In contrast, methane, carbon dioxide, and nitrous oxide are all naturally occurring gases. Methane is produced by both natural processes, such as the decay of organic matter and the digestive processes of ruminant animals, and human activities, primarily through agriculture and fossil fuel extraction. Carbon dioxide is released during respiration, volcanic eruptions, and various natural processes, while nitrous oxide is generated from soil microbial processes and certain marine environments, among other sources. Thus, it is clear that among the options provided, only CFCs are not naturally occurring gases.