The correct answer is option 3, Ozone.

To understand why ozone is not a primary pollutant of burning fossil fuels, we need to first understand what primary pollutants are. Primary pollutants are those that are emitted directly into the atmosphere from sources such as vehicles, industries, and power plants, without undergoing any chemical changes. These pollutants include sulphur dioxide, nitrogen dioxide, and carbon monoxide.

Sulphur dioxide (option 1) is produced when fossil fuels that contain sulfur, such as coal and oil, are burned. Nitrogen dioxide (option 2) is a result of the combustion of fossil fuels in the presence of nitrogen from the air. Carbon monoxide (option 4) is formed when fossil fuels are not burned completely.

On the other hand, ozone (option 3) is not emitted directly from burning fossil fuels. It is a secondary pollutant formed in the atmosphere through complex reactions involving sunlight, nitrogen oxides (which includes nitrogen dioxide), and volatile organic compounds (VOCs) released from various sources including vehicle emissions, industrial processes, and chemical solvents.

Therefore, the correct answer is option 3, as ozone is not a primary pollutant of burning fossil fuels.