UPSC CSE Prelims

Correct Option: A Explanation: Photochemical smog forms when sunlight drives reactions of nitrogen oxides and volatile organic compounds, yielding ozone and secondary oxidants including PAN. Option-wise Analysis: A: Correct - Correct in the intended exam framing: NO₂, ozone and PAN are signature components of sunlit photochemical smog. B: Incorrect - Wrong: carbon monoxide and oxygen alone with PAN do not describe the principal NOₓ–VOC smog system. C: Incorrect - Wrong: CO, CO₂ and NO₂ at low temperature do not produce classic sunlight-driven oxidant smog. D: Incorrect - Wrong: the evening timing contradicts the strong sunlight needed for peak photochemical oxidant formation. Review Note: Scientifically, NO₂ and VOCs react under sunlight to form ozone and PAN; ozone and PAN are primarily products rather than the full original set of reactants. The supplied A is the closest option.