Which one of the following—species is stable in aqueous solution?

Species is stable in aqueous solution is ${\mathrm{MnO}}_4^{2-}$.

${\mathrm{MnO}}_4^{3-}$ is unstable in aqueous solution due to the unstable +5 oxidation state of $\mathrm{Mn}$.

${\mathrm{MnO}}_4^{2-}$ is stable in aqueous solution because $\mathrm{Mn}$ +6 oxidation state is steady.

Hence, option B is correct.