AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
(C) Lemon juice
Acids turn blue litmus red. Lemon juice is acidic in nature, whereas NaOH, KOH, and baking soda solution are basic and would turn red litmus blue.
The key fact from the textbook (Introduction, Chapter 2) is: "acids change the colour of blue litmus to red, whereas bases change the colour of red litmus to blue." NaOH, KOH, and baking soda (NaHCO₃) are all bases/alkaline — they turn red litmus blue. Only lemon juice is acidic (contains citric acid), so it turns blue litmus red. Always remember: Acid → Blue to Red; Base → Red to Blue.