Acid rain is caused by the pollution of environment by

Acid rain forms when sulphur dioxide (SO₂) and oxides of nitrogen (NO_x) released mainly from coal-fired power plants, smelters and vehicles react in the atmosphere with water vapour to form sulphuric and nitric acids. Option D is the intended answer. Strictly, the nitrogen oxide that matters is NO and NO₂ rather than nitrous oxide (N₂O), which is mainly a greenhouse gas, so read the option as 'nitrogen oxides'.

• A is wrong because nitrogen is inert and CO₂ is not a significant acid-rain precursor.
• B is wrong because CO is a toxic gas, not an acid precursor, and CO₂ only adds to global warming.
• C is wrong because ground-level ozone is a pollutant but does not cause acid rain.

The core concept is transboundary air pollution and its ecological and material damage. For Mains, link it to the Air (Prevention and Control of Pollution) Act 1981, the sulphur emission norms for thermal plants, and the National Clean Air Programme launched in 2019.

Trap: aspirants pick A or B because CO₂ is the most famous gas, but acid rain is driven by sulphur and nitrogen oxides, not carbon gases.