One mole ${MnO}_4^{2-}$ ion in neutral solutions under goes disproportion to give

In a neutral aqueous solution,${MnO}_4^{2-}$ disproportionate into ${MnO}_4^{-}$ and ${MnO}_2$.

${MnO}_4^{2-}+\frac{2}{3}{H}_{2}O\rightleftharpoons \frac{2}{3}{MnO}_4^{-}+\frac{1}{3}{MnO}_2\downarrow +\frac{4}{3}{OH}^{-}$

1 mole of manganate ion, ${MnO}_4^{2-}$ decommposes to 2/3 moles of permanganate ion, ${MnO}_4^{-}$ and 1/3 moles of manganese dioxide, ${MnO}_2$.

Therefore, option A is correct.