The answer is C (\(\text{NO}_2\)).
• Photochemical smog: nitrogen oxides (\(\text{NO}_x\)) such as \(\text{NO}_2\) react with volatile organic compounds (VOCs) in the presence of sunlight, forming a brownish haze over urban areas.
• Acid rain: \(\text{NO}_2\) dissolves in rainwater and is oxidised to nitric acid: \(4\text{NO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HNO}_3\).
So \(\text{NO}_2\) takes part in both.
• A is wrong: CO is a toxic pollutant but causes neither photochemical smog nor acid rain.
• B is wrong: \(\text{CO}_2\) is a greenhouse gas; it causes neither photochemical smog nor acid rain.
• D is wrong: \(\text{SO}_2\) causes acid rain (forming \(\text{H}_2\text{SO}_4\)) but does not take part in forming photochemical smog.
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