Fossil fuel combustion increases the acidity of rain because

carbon dioxide dissolves in rainwater

sulfuric acid and nitric acid dissolve in rainwater

tiny particles of carbon dissolve in rainwater

sulfur dioxide and nitrogen oxides dissolve in rainwater

The second answer would be correct.

When fossil fuels combust, compounds such as NOₓ (Nitrogen Oxides) and SO₂ (Sulfur Dioxide). These compounds can then react with the environment to form the acids sulfuric acid and nitric acid, which, when reacted with water, release H₊ ions. The higher the concentration of H₊ ions, the more acidic the solution.

sulfur dioxide and nitrogen oxides dissolve in rainwater

Explanation:

The burning of fossil fuels leads to the emission of sulphur dioxide and nitrogen oxides. These are an acid gases. They dissolve in water to give acids. These acids lower the pH of rain water.

Hence in areas with high emission of burning of fossil fuels, the rain fall is likely to be acidic. Acid rain has many deleterious effects such as damage to roofing sheets, sculptures and paints of buildings as well as affecting the pH of natural water bodies.