The rapid change of $pH$ near the stoichiometric point of an acid-base titration is the basis of indicator detection. $pH$ of the solution is related to ratio of the concentration of the conjugated acid(HIn) and base (In) form of the indicator by the expression

Question

(a) $log \frac{[In^-]}{[HIn]} = pH - pK_{In}$

(b) $log \frac{[In^-]}{[HIn]} = pH_{In} - pH$

(c) $log \frac{[HIn]}{[In^-]} = pK_{In} - pH$

(d) $log \frac{[HIn]}{[In^-]} = pH - pK_{In}$