Acid rain happens because burning of fossil fuels release oxides of carbon, nitrogen and sulphur in the atmosphere.
Explanation: When fossil fuels are burned, they release pollutants like sulfur dioxide and nitrogen oxides into the air, which react with water vapor to form acidic compounds that fall as rain.
Why the other options are incorrect:
(a) sun leads to heating of upper layer of atmosphere:
While the sun’s heat contributes to atmospheric warming, it is not the direct cause of acid rain.
(c) electrical charges are produced due to friction amongst clouds:
Friction between clouds can generate electricity, but this is not related to the formation of acid rain.
(d) earth atmosphere contains acids:
The Earth’s atmosphere naturally contains some acids, but the levels are significantly lower than in acid rain, which is caused by human pollution.