b
For example, an indicator \(\left( HI _{ n }\right)\) can dissociate in the solution following way.

\(HIn \rightleftharpoons H ^{+}+\operatorname{In}^{\ominus}\)

\(\therefore E _{\text {Equilibrium Constant }}, K _{ In }=\frac{\left[ H ^{+}\right]\left[ In ^{-}\right]}{[ HIn ]}\)

\(\therefore pK _{\text {In }}= pH -\log \frac{\left[ In ^{\ominus}\right]}{[ HIn ]}\)

\(\Rightarrow \log \frac{\left[\operatorname{In}^{\ominus}\right]}{[ HIn ]}= pH - pK _{ In }\)