The correct option is D O3
Secondary pollutants are pollutants that are not emitted directly from a source. For example: ozone is formed through chemical reactions of other molecules already in the air. So it is considered as a secondary pollutant. These are formed by chemical reaction amongst primary pollutants. For example, photochemical smog, London smog (Classical smog), PAN (Peroxy acetyl nitrate), O3 (Ozone).
So, the correct answer is "option D"