Which among the following species acts as a self-indicator?

In the presence of a reducing agent, ${{\mathrm{MnO}}_4}^{-}$​ acts as self-indicator and changes colour from pink to colourless. Because in ${{\mathrm{MnO}}_4}^{-}$oxidation state of $\mathrm{Mn}$ is $+7$ and it is in highest oxidation state. Thus will tend to get reduced and easily takes electrons and being a charge transfer complex, shows intense colour. So that's why it is act as self indicator.