Detailed Explanation:

Burning of fossil fuels (coal, petroleum, natural gas) in power plants, vehicles, and industries releases sulfur dioxide (SO₂), nitrogen oxides (NOₓ), carbon monoxide (CO), and particulate matter (PM2.5) directly into the atmosphere.

These pollutants are the primary cause of outdoor air pollution, leading to smog, respiratory diseases, and acid rain, while other options mainly affect soil, water, or indirectly impact air quality.