Which of the following is a natural cause of air pollution?

Air pollution can be caused by both natural and human-made sources. One of the natural causes of air pollution is volcanic eruptions, which release various pollutants, including sulfur dioxide, carbon dioxide, and particulate matter. 

These pollutants can have adverse effects on air quality and human health. Unlike human-made sources, natural causes of air pollution, such as volcanic eruptions, cannot be controlled by humans. 

Therefore, the correct answer is volcanic eruptions.