which factor is a major contributor to outdoor air pollution?\nradon\nfossil fuels\nultrafine…

Brief Explanations:

Fossil - fuel combustion is a major source of outdoor air pollutants. Radon is more of an indoor pollutant. Ultrafine particles and carbon - monoxide are pollutants but fossil fuels are the major contributor as their burning releases a wide range of pollutants including ultrafine particles and carbon - monoxide.

Answer:

fossil fuels