Primary pollutants are pollutants that are directly emitted into the atmosphere from a source, while secondary pollutants are formed in the atmosphere from reactions between primary pollutants. Of the given options, Carbon Monoxide (CO), Nitrogen Dioxide (NO2), Sulphur Dioxide (SO2), and Particulate Matter (PM10) are all primary pollutants emitted directly from sources such as vehicles, power plants, and industrial processes. Therefore, the correct answer is Ozone (O3), as it is a secondary pollutant formed in the atmosphere through reactions between other pollutants in the presence of sunlight.