Correct option (b) dry acid deposition
Sulfur and nitrogen oxides emitted into the atmosphere react to form compounds that are transported long distances and are subsequently deposited to the Earth's surface in wet and dry forms. Although the term "acid rain" is widely recognised, the dry deposition portion ranges from 20 to 60 percent of total pollutant deposition, and represents the particulate and gaseous forms. A more accurate description of the overall process is acid deposition rather than acid rain. Dry deposition occurs when sulfur dioxide and nitrogen oxides react, but not with water. It settles out of the atmosphere as particle or gases.