Rain is called acid rain when pH is below

• Acid rain is rain that is acidic or has an extremely low pH. 
• Sulfuric and nitric acids are produced when sulfur dioxide and nitrogen oxides are released into the atmosphere and react with water and oxygen. 
• Acid rain is formed when these acids react with water.
• Normal rainwater has a pH of 5-6 and is mildly acidic. 
• Acid rain occurs when the pH level of rainwater goes below of pH 5-6 .

Hence, option D is the correct answer.