Which of the following acts as a self–indicator?

Indicators are substances that tell or indicate whether the substance is acidic or basic in nature by showing some change in its physical properties.

Mostly indicators like phenolphthalein, methyl orange or litmus change their colour.

${\mathrm{KMnO}}_4$ works as a self-indicator and transforms from pink to colourless in the presence of a reducing agent. $\mathrm{Mn}$ is in the highest oxidation state, +7, in ${\mathrm{MnO}}_4^{-}$ because of this. As a result, it tends to get reduced, readily accepts electrons, and exhibits strong colour since it is a charge-transfer complex. 

After titration, ${\mathrm{MnO}}_2$ is in the +4 state and is colourless since there has been no electronic charge transfer.

Hence, option (B) is the correct answer.