In which situation will phenolphthalein turn from colourless to pink?

Correct answer: When base is added to an acidic solution until it becomes basic

Phenolphthalein is colourless in acidic and generally neutral conditions, but it becomes pink in a basic medium. Therefore, when base is added to an acidic solution in sufficient amount to make the final solution basic, the indicator changes from colourless to pink. Adding acid to a base would remove the pink colour as the medium becomes acidic. The salt choices do not necessarily create a basic solution, so option A is correct.