Blue litmus turns red in acid. What does this identify?

Correct answer: Acidic nature

Litmus is an acid–base indicator that gives a characteristic colour change. When blue litmus paper turns red, the tested substance has an acidic nature, because acids change blue litmus to red. Thus option A is correct. A base generally turns red litmus blue, while a neutral solution normally causes no such acid-indicating change; metallic nature is not identified by this test.