Given below are two statements:one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Phenolphthalein is a pH dependent indicator, remains colourless in acidic solution and gives pink colour in basic medium.
Reason R: Phenolphthalein is a weak acid. It doesn't dissociate in basic medium.
In the light of the above statements, choose the most appropriate answer from the options given below

Phenolphthalein is indeed a pH-dependent indicator, remaining colourless in acidic solutions (pH < 8.2) and turning pink in basic solutions (pH 8.2 to 12). Therefore, this statement is true.

Regarding the reason, it is correct that phenolphthalein is a weak acid, but the claim that it does not dissociate in basic medium is false. In basic medium, phenolphthalein actually does dissociate. The undissociated form (HIn) is colourless, and when it dissociates in basic medium to form $$In^-$$ (the conjugate base), it turns pink. The basic medium promotes the dissociation of this weak acid, shifting the equilibrium toward the ionized form.

$$HIn \rightleftharpoons H^+ + In^-$$

(colourless) $$ $$ (pink)

Consequently, the reason that it "doesn't dissociate in basic medium" is incorrect.

Since Assertion A is true but Reason R is false, the correct answer is Option C.