Acid rain is primarily caused by chemicals released into the air by factories, particularly sulfur dioxide (SO2) and nitrogen oxides (NOx). These pollutants can react with water vapor in the atmosphere, leading to the formation of sulfuric and nitric acids, which then fall to the ground as acid rain.
While factors like drought or over-fertilization of farmlands can impact soil health and erosion, they are not direct causes of acid rain. Sudden cloud formation does not directly contribute to acid rain either. Thus, the correct answer to your question is:
chemicals released into the air by factories.