Final answer:
The most acidic 20% of all storms would have a pH lower than the average for acid rain, which means the pH level would likely be around 4.5.
Step-by-step explanation:
The pH scale is used to determine the acidity or alkalinity of a solution.
Normal rainwater has a pH between 5 and 6 due to the formation of carbonic acid from dissolved CO₂ in the atmosphere. Acid rain, however, has a pH of less than 5, indicating higher levels of acidity due to the presence of other nonmetal oxides forming stronger acids such as sulfuric and nitric acids.
Given that the most acidic rain recorded in the U.S. has a pH of about 4.3 as of 2000, and that the top 20% of the most acidic storms would have a pH lower than the average, the answer to the question 'What is the pH level for the most acidic 20% of all storms?' would be (a) pH 4.5.