A student dipped blue litmus paper into a colourless solution, and it turned red. Which conclusion about the solution is correct?

Correct answer: The solution is acidic

Blue litmus turning red is the characteristic indicator response of an acidic solution. Acids provide hydrogen ions in water, which cause this colour change. A basic solution generally turns red litmus blue, while a neutral solution produces no litmus change. The observation does not prove that the liquid contains only salt, because salts may be acidic, basic, or neutral. Thus option A is correct.