Acid rain is due to air pollution by

Acid rain forms when sulphur dioxide (SO₂) and nitrogen oxides (NO_x) react with water vapour and oxygen in the atmosphere to form sulphuric and nitric acids. The main sources are thermal power plants burning high-sulphur coal, smelters and industry for SO₂, and vehicles and combustion processes for NO_x. Strictly, the option says nitrous oxide, but the examiner means nitrogen oxides. Nitrous oxide (N₂O) itself is mainly a greenhouse gas and ozone-depleting substance, so read the option as NO_x.

• A) Carbon dioxide: CO₂ forms carbonic acid, but it is too weak to count as acid rain. Normal rain is already slightly acidic, around pH 5.6.
• B) Carbon monoxide: CO is a toxic gas, but it is not an acid precursor. It gets oxidised to CO₂.
• C) Methane: CH₄ is a greenhouse gas that contributes to warming and ground-level ozone. It has no role in acid rain.

This sits under Environment, air pollution and its effects, and links to GS-III Mains questions on pollution control, emission norms and the damage acid rain does to forests, lakes and monuments like the Taj Mahal.

Trap: Aspirants pick CO₂ because carbonic acid is a real acid. The exam term refers only to the strong acid-forming oxides, SO₂ and NO_x.