PAN (Peroxyacetyl nitrate) is a secondary gaseous compound found in _?

Answer: [C] Photochemical smog

Third option is the correct answer
Photochemical smog is the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere, which leaves airborne particles and ground-level ozone.

This question is a part of GKToday's Integrated IAS General Studies Module