The pink colour of phenolphthalein in alkaline medium is due to  

Phenolphthalein is an acid-base indicator that appears colorless in acidic solutions and pink in basic solutions. Its pH range is 8.2 (colorless) -10.0 (pink).Phenolphthalein is a weak acid with a tendency to lose ${\mathrm{H}}^{+}$ ions in solution. The phenolphthalein ion is pink in colour, and the phenolphthalein molecule is colourless.

The structure of phenolphthalein in an acidic environment has ${\mathrm{H}}^{+}$. As we add the base, the solution becomes basic beyond the endpoint. In an alkaline environment, ${\mathrm{OH}}^{-}$ ions neutralize this proton and the structure of phenolphthalein becomes an anion. 

This anionic structure also known as the quinoid structure is resonance stabilized. This quinoid structure, which is the anionic form, is responsible for the pink color of phenolphthalein.

Hence, the correct option is (B).