Which one of the following will turn red litmus blue?

Explanation: A red litmus paper turns blue when exposed to a basic (alkaline) solution.

• Vinegar is acidic (pH < 7), so it will not turn red litmus blue; instead, it will keep it red.
• Baking soda solution, on the other hand, is basic (alkaline) with a pH above 7. Therefore, it will turn red litmus paper blue.
• Lemon juice is also acidic (pH < 7), so it will keep the litmus paper red.
• Soft drinks are generally acidic (e.g., cola), and thus will not turn red litmus blue; they will also keep it red.

Thus, baking soda solution is the correct option because it is alkaline and will turn red litmus paper blue.