Which of the following is a common air pollutant?

Carbon monoxide is recognized as a common air pollutant due to its presence as a byproduct of incomplete combustion of fossil fuels. It is an odorless, colorless gas that can be harmful to human health and the environment. In urban areas, carbon monoxide often comes from vehicles and industrial processes, contributing to air pollution and potentially leading to respiratory issues and other health complications when inhaled in significant amounts.

The ozone layer, while crucial for absorbing harmful ultraviolet radiation from the sun, is not classified as a common air pollutant. Instead, it serves as a protective barrier in the upper atmosphere.

Chlorofluorocarbons (CFCs), once widely used in refrigeration and aerosol propellants, have been phased out due to their role in ozone depletion, but they are not typical air pollutants in the same way as carbon monoxide.

Methane, a potent greenhouse gas, does contribute to climate change and can be found in the atmosphere as a result of agricultural and industrial practices, but it is not typically classified as a common air pollutant in the context of urban air quality. Thus, carbon monoxide stands out as the primary example of a common air pollutant among the options provided.