Concept:Phenolphthalein is an acid-base indicator that changes colour depending on the pH of the solution.Explanation:Phenolphthalein remains colourless in acidic and neutral solutions.In an alkaline solution, it turns pink.At the end-point of an acid-base titration, the solution becomes slightly alkaline if phenolphthalein is used.Therefore, the indicator shows its alkaline colour, which is pink.Answer:C. pink