What effect does an acidic solution have on blue litmus paper?

Correct answer: Turns it red

Litmus is a natural acid-base indicator. In an acidic medium, blue litmus changes to red because the indicator responds to the presence of acidic ions. This red colour is a characteristic test for acidity, although it does not by itself measure the exact pH or strength of the acid. A basic solution generally turns red litmus blue, while green or no change is not the standard result for blue litmus in acid. Thus A is correct.