When phenolphthalein is added to$NaOH$, the colour of the solution will become _________
(a)- Colorless
(b)- Red
(c)- Pink
(d)- Yellow

Hint: In a neutral solution, the phenolphthalein would give a light pink colour. When phenolphthalein is added to the base the colour of the solution intensifies and if it is added to an acidic solution the colour will fade away and become colourless.

Complete step by step answer:
Phenolphthalein is a chemical compound that is used as an indicator in acid-base titrations.
The chemical formula of phenolphthalein is${{C}_{20}}{{H}_{14}}{{O}_{4}}$.
Structure of phenolphthalein is given below:


In acidic medium the colour of phenolphthalein is colourless and in the basic medium, it has a very dark pink colour.
The reaction with acid and base is given below:

It mostly dissolves in alcohol and is slightly soluble in water.
When dissolved in water it can lose ${{H}^{+}}$ions and form phenolphthalein ions. The phenolphthalein ion is pink and the phenolphthalein molecule is colourless.
When the phenolphthalein is dissolved in the base the ionization starts and the formation of phenolphthalein ion increases, this makes the solution dark pink.
But when the acid is added, the phenolphthalein ion gets converted into phenolphthalein molecule making the solution colourless.
Earlier phenolphthalein was used as a laxative in the medical field. A laxative is used for the treatment of constipation. But it is being removed as a laxative because it is carcinogenic. It restricts the function of calcium ions in the body.
So, $NaOH$is a base, and the phenolphthalein gives pink colour in the basic solution.

Hence, the correct answer is an option (c)- Pink.

Note: If the phenolphthalein is added to a very acidic solution then it would give an orange colour. When phenolphthalein is added to a highly basic solution then the colour changes from pink to colourless because it forms$\ln {{(OH)}^{3-}}$ ions. It is used for the study of reaction kinetics.