If an acidic indicator HIn ionizes as $\mathrm{HIn} \rightleftharpoons {\mathrm{H}}^{+} + {\mathrm{In}}^{-}.$ To which maximum pH value its solution has distinct colour characteristic of HIn.

• A

  ${\mathrm{pK}}_{\mathrm{in}}-1$

• B

  ${\mathrm{pK}}_{\mathrm{in}}\pm 1$

• C

  ${\mathrm{pK}}_{\mathrm{in}}$

• D

  7