a
\(MnO _{4}^{2-}\) disproportionates in a neutral or acidic solution to give \(MnO _{4}^{-}\)and \(Mn ^{+4}\)

\(3 MnO _{4}^{2-}+3 H ^{+} \longrightarrow 2 MnO _{4}^{-}+ MnO _{2}+2 H _{2} O\)

O.S. of \(Mn\) in \(MnO _{4}^{-}=+7\)

O.S. of \(Mn\) in \(MnO _{2}=+4\)

\(\text { difference }=3\)