On adding acid to a solution, the pink colour of phenolphthalein disappears. What was the initial nature of the solution?

Correct answer: Basic

Phenolphthalein is colourless in acidic or neutral conditions but turns pink in a sufficiently basic solution. If adding acid makes the pink colour disappear, the solution must initially have contained enough base to produce that colour. The acid lowers the alkalinity through neutralisation. Thus option B is correct; the other choices cannot explain the initial pink indicator colour.