which of the following substances cannot be a primary air pollutant? carbon monoxide smog sulfur dioxide nitrogen dioxide

 Smog

A primary air pollutant can be defined as a pollutant which is released directly into the air from the source of it's origin. It includes gases like sulphur dioxide, ammonia, particulate matter, volatile organic matter, nitrogen oxides and carbon monoxide.  A secondary air pollutant is a pollutant that is not directly released from a source into the atmosphere, instead it can be defined as aggregation of primary pollutants with the atmospheric gases.

Smog is an example of secondary pollutant, it cannot be included in the primary pollutants. It is formed by combination of primary pollutant with oxygen, water and hydrocarbon present in the atmosphere.