The main reason for acid rain is the dissolution of sulphur and nitrogen oxides in rain, which is stated in option 1. Sulfur and nitrogen oxides are released into the atmosphere primarily through industrial processes such as burning fossil fuels and from natural sources like volcanic eruptions. These gases react with water, oxygen, and other chemicals in the atmosphere to form sulfuric acid, nitric acid, and other acidic compounds.

Option 2, the dissolution of minerals in rain, is not the main reason for acid rain. While minerals can dissolve in rainwater, they do not contribute significantly to the acidity of rain.

Option 3, the dissolution of dust particles in rain, is also not the main cause of acid rain. Dust particles generally do not contain acidic substances that can cause a significant increase in the acidity of rain.

Option 4, the dissolution of soil solution in rain, is not the main cause of acid rain. Soil solutions usually do not contain high levels of sulphur and nitrogen oxides that can contribute to the acidity of rain.

Overall, option 1 correctly identifies the main reason for acid rain, while the other options do not capture the primary cause effectively.