Smog is essentially caused by the atmospheric presence of:

The answer is D. Smog is a mix of smoke and fog, and its main drivers are oxides of nitrogen (NO and NO2) and oxides of sulphur (mainly SO2), along with particulate matter. Sulphur dioxide and particulates produce classical smog, the London type, mostly from coal burning. Nitrogen oxides and volatile organic compounds, acted on by sunlight, produce photochemical smog, the Los Angeles type, in which ozone forms as a secondary product.

• A: Oxygen is the normal air baseline, and ozone is a product of photochemical smog rather than its root cause.
• B: Nitrogen is about 78 per cent of dry air and is inert under these conditions, so it cannot be the causal agent.
• C: Oxygen and nitrogen make up clean air. They are the baseline, not the pollution.

Concept: air pollutants and their sources. This links to the Air (Prevention and Control of Pollution) Act, 1981, and the National Clean Air Programme (2019), which targets SO2, NOx and PM together. For Mains, it connects to urban health and the vehicle-versus-industry debate in Delhi-NCR.

Trap: Candidates pick A because ozone gets named often in smog discussions, but it is a secondary product, not the primary cause the question asks for.