Photochemical smog is a type of air pollution that forms when primary pollutants react in the atmosphere under the influence of sunlight. Understanding the key components is crucial for identifying the correct answer.

Identifying Photochemical Smog Pollutants

Photochemical smog primarily involves the interaction of sunlight with specific pollutants released into the atmosphere, mainly from vehicle emissions and industrial processes. The key culprits are:

• Oxides of nitrogen (NO_x): These are released from burning fossil fuels. They react with sunlight and oxygen to initiate the formation of other smog components.
• Unburnt hydrocarbons (VOCs): Also emitted from vehicles and industrial sources, these organic compounds react with NO_x in the presence of sunlight.
• Ozone (O₃): This is a major secondary pollutant formed during the reactions between NO_x and hydrocarbons under sunlight. It is a significant component of photochemical smog.

Excluding Non-Contributing Pollutants

Sulphur dioxide (SO₂), while a significant air pollutant, is primarily associated with *classical smog* (or London smog), which forms due to the reaction of sulphur-based fuels with atmospheric moisture and particulate matter. It is not a direct driver of photochemical smog formation.

Conclusion

Based on the formation process, the air pollutants responsible for photochemical smog are oxides of nitrogen, ozone, and unburnt hydrocarbons. Therefore, the correct combination is (a), (b), and (c).