Methyl orange is red and phenolphthalein is colourless in a solution. What should be added to neutralise it?

Correct answer: Sodium hydroxide solution

Indicator behaviour shows that the solution is acidic: methyl orange is red in a strongly acidic medium, and phenolphthalein remains colourless in acidic and neutral media. To neutralise an acid, a suitable base must be added so that H⁺ and OH⁻ react to form water: H⁺ + OH⁻ → H₂O. Sodium hydroxide solution supplies OH⁻ ions and is therefore the correct choice, option B. Adding more acid would increase acidity, while sodium chloride or water alone does not provide the required neutralising base.