1. Check Assertion A

Phenolphthalein is an acid–base indicator.

• In acidic medium, it remains colourless.
• In basic medium, it turns pink.

This is a standard property of phenolphthalein.

So, Assertion A is true.

2. Check Reason R

Reason says:

Phenolphthalein is a weak acid. It doesn't dissociate in basic medium.

Let us analyze this carefully.

Phenolphthalein behaves as a weak acid indicator and can be represented as:

$\mathrm{HPh \rightleftharpoons H^+ + Ph^-}$

• The undissociated form is colourless.
• The dissociated/anionic form in basic medium is pink.

In basic medium, due to removal of $\mathrm{H^+}$ by $\mathrm{OH^-}$, the equilibrium shifts to the right, so phenolphthalein does dissociate more in basic medium, producing the pink ionized form.

Hence the statement "It doesn't dissociate in basic medium" is false.

Therefore, Reason R is false.

3. Select the correct option

• Assertion A: True
• Reason R: False

So the correct option is:

$\boxed{\text{C: A is true but R is false}}$

4. Comparison with stored correct answer

Stored correct answer = C

Our derived answer = C

So, the derived answer agrees with the stored correct answer.