The correct answer is Dissolution of sulphur and nitrogen oxides in rain.Explanation:Normally rainwater has a pH of 5.6 due to the presence of H+ ions formed by the reaction of rainwater with carbon dioxide present in the atmosphere.Acid rain occurs when the pH of rainwater falls below 5.6.Acid rain is the process by which acid from the atmosphere is deposited on the earth's surface. Oxides of nitrogen and sulphur which are acidic in nature can be blown by wind along with solid particles in the atmosphere and finally settle down either on the ground as dry deposition or in water, fog and snow as wet deposition.Acid rain is harmful to agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth.It causes respiratory ailments in human beings and animals.