What is the nature of a solution that turns blue litmus paper red?

Correct answer: Acidic

Litmus is an acid–base indicator with a characteristic colour change. Blue litmus turns red in an acidic medium because the indicator responds to the presence of excess hydrogen ions. A basic solution generally turns red litmus blue, while a neutral solution does not produce this acidic change. Therefore the solution described is acidic, making option A correct; “salty” describes neither a definite indicator response nor a guaranteed pH.