Phenolphthalein is added to a colourless solution and it turns pink. On adding acid, the colour disappears. What does this show?

Correct answer: The original solution was basic and became neutralised by acid

Phenolphthalein is colourless in an acidic or neutral medium and pink in a sufficiently basic medium. The initial pink colour therefore shows that the original solution was basic. Adding acid consumes hydroxide ions through neutralisation, lowering the pH; when the solution leaves the indicator’s pink transition range, the colour disappears. Thus A is correct. The observation does not show that the original solution was strongly acidic.