Answer: sulfur oxides
Step-by-step explanation:
Smog is a kind of severe air pollution which results due to accumulation of pollutants like nitrogen oxides, ozone, smoke, sulphur oxides and other particulates. These pollutants are added to the atmosphere through coal combustion, industrial emission, fires in agriculture field and forests and lastly because of photochemical reactions.
Among the gaseous pollutants the sulfur oxides are present in high amount as compared to other gases. As the sulfur oxides are released by the burning of fossil fuels. These oxides are the reducing agents which reacts with the oxygen present in the atmosphere result in the formation of a photochemical smog.