Option 1: Sulphur dioxide (SO2) is a gas that is primarily released into the air by burning fossil fuels, such as coal and oil. It is a major contributor to air pollution and can cause respiratory problems. However, it is not specifically associated with the formation of brown air in traffic congested cities.

Option 2: Nitrogen oxide (NOx) is a group of gases that are released into the air when fuel is burned at high temperatures. It is commonly produced by vehicles and power plants. Nitrogen oxide is a major contributor to air pollution and is associated with the formation of brown air in traffic congested cities. It reacts with other pollutants in the presence of sunlight to form ozone, particulate matter, and other harmful compounds.

Option 3: Carbon dioxide (CO2) is a greenhouse gas that is primarily released into the atmosphere through the burning of fossil fuels. While it is a major contributor to climate change, it is not directly associated with the formation of brown air in traffic congested cities.

Option 4: Carbon monoxide (CO) is a colorless and odorless gas that is produced by the incomplete combustion of fossil fuels. It is a common pollutant in urban areas, particularly in traffic congested cities. While