`E_(Mn^(3+)//Mn^(2+))^@`is highly positive than that of `E_(Cr^(3+)//Cr^(2+))^@orE_(Fe^(3+)//Fe^(2+))^@` because

Answer

(a) Much larger third ionisation energy of `Mn` (Where the required change is `d^(5) to d^(4)` ) is mainly responsible for this. This also explains why the `+3` state of `Mn` is of little importance.
When a perticular oxidaton state become less stable relative to other oxidation state lower and one higher, it is said to undergo disproportionation . For example , Manganese(VI) become unstable relative to `3Mn ^viO_(4)^(2-)+4H^(+) to 2Mn ^(VII)O_(4)^(-)+Mn^(IV)O_(2)+2H_(2)O`