Photochemical smog is a resultant of the reaction among

Photochemical smog is a complex mixture of pollutants formed through the interaction of sunlight with certain primary pollutants. Here's a breakdown:

Correct Answer (A): NO2, O3 and peroxyacetyl nitrate (PAN) in the presence of sunlight. This is the correct answer because these are the key components formed during photochemical reactions. NO2 absorbs sunlight and breaks down, releasing oxygen atoms that combine with O2 to form ozone (O3). PAN is a strong oxidant formed from the reaction of unburnt hydrocarbons with NOx in sunlight, and is a major contributor to the irritating effects of smog.

Incorrect Option (B): CO, O2 and peroxyacetyl nitrate in the presence of sunlight. While PAN is involved, CO doesn't directly participate in the formation of photochemical smog. CO is a pollutant, but not a primary reactant in the smog-forming process.

Incorrect Option (C): CO, CO2 and NO2 at low temperature. Photochemical smog is formed by reactions initiated by sunlight (high energy), not at low temperatures. CO2 is a greenhouse gas but not a direct component of smog formation.

Incorrect Option (D): high concentration of NO2, O3 and CO in the evening. Smog formation is highest during the day when sunlight is available. Concentrations may persist into the evening, but the formation process requires sunlight.

Therefore, understanding the role of sunlight and the key reactants (NO2, O3, and PAN) is crucial to answering this question.