In which situation will blue litmus turn red?

Correct answer: When placed in vinegar solution

Vinegar contains acetic acid, so its aqueous solution is acidic. Acids turn blue litmus paper red because the indicator changes colour in an acidic environment; therefore option A is correct. Sodium hydroxide, soap solution, and lime water are basic substances or solutions and generally turn red litmus blue rather than changing blue litmus to red. Identifying the nature of each solution before applying the indicator rule avoids confusion.