Photochemical smog is a resultant of the reaction among.

Option A is correct. Photochemical smog forms when oxides of nitrogen and volatile organic compounds react under strong sunlight. NO₂ photolyses to give NO and atomic oxygen, which combines with O₂ to form ground-level ozone (O₃). Peroxyacetyl nitrate (PAN), formed from peroxyacetyl radicals and NO₂, is a secondary pollutant and a strong eye irritant. This is the Los Angeles type of smog, and it is an oxidising smog.

• B is wrong: PAN belongs to the smog, but CO and O₂ are not its characteristic reactants. O₂ is not a pollutant.
• C is wrong: This describes London-type (winter or reducing) smog. That smog is driven by coal burning, SO₂, particulates and cold humid conditions, not by photochemistry.
• D is wrong: Photochemical smog peaks around midday and afternoon, when sunlight is strongest. Evening is not the time of formation, and CO is not a key precursor.

Connect this to the Environment syllabus: the reason ozone is a ground-level pollutant here, though it protects us in the stratosphere, is a good Mains point on the role of sunlight and precursors.

Trap: Aspirants pick B because PAN is the recognisable name and CO sounds like a pollutant. Remember that CO is not a photochemical smog precursor.