Answer: donates protons in a proton transfer reaction.
Step-by-step explanation:
According to the Arrhenius concept, an acid is a substance that ionizes in the water to give hydrogen ion and a base is a substance that ionizes in the water to give hydroxide ion .
According to the Lewis concept, an acid is defined as a substance that accepts electron pairs and base is defined as a substance which donates electron pairs.
Phenolphthalein is an indicator which produces pink colour on adding into a base and turns colourless on adding to an acid.
Thus the only correct option is an acid donates protons in a proton transfer reaction.