The correct answer is option 4 - Smoke. Smoke is not considered a gaseous air pollutant. Let`s break down each option to understand why smoke doesn`t fall into the category of gaseous air pollutants.

1. Oxides of sulphur: Oxides of sulphur, such as sulphur dioxide (SO2) and sulphur trioxide (SO3), are gaseous air pollutants. They are released into the air during the combustion of fossil fuels, industrial processes, and volcanic eruptions.

2. Oxides of nitrogen: Oxides of nitrogen, such as nitrogen dioxide (NO2) and nitric oxide (NO), are also gaseous air pollutants. They are primarily generated from the burning of fossil fuels, industrial activities, and vehicle exhaust.

3. Hydrocarbon: Hydrocarbons, such as methane and volatile organic compounds (VOCs), are a group of gaseous air pollutants. They are released from vehicle emissions, industrial processes, and natural sources like vegetation.

4. Smoke: Smoke is not a gaseous air pollutant. It consists of solid particles and liquid droplets produced by incomplete combustion or burning of organic materials, such as wood, coal, or tobacco. While smoke can contribute to air pollution,