A student dips red litmus paper into a soap solution. What change will occur in the red litmus paper?

Correct answer: It will turn blue.

Soap usually contains salts of weak acids and strong bases, and its aqueous solution is alkaline. According to the litmus rule, an alkaline solution changes red litmus paper to blue, so option A is correct. In an acidic or neutral solution, red litmus would remain red; it would not become colourless or change twice.