Why is `Cr^(2+)` acts as reductant and `Mn^(3+)` as oxidant eventhough both have `d^4` configuration ?

Explanation

`Cr^(2+) (d^(3))` is less stable than `Cr^(3+) (d^(3))` because the latter having a half-filled `t_(2g)` level. Thus `Cr^(2+)` oxidises easily to `Cr^(3+)` and acts as reductant.
`Mn^(3+) (d^(4))` is less stable than `Mn^(2+) (d^(5))` because the latter has half-filled d-subshell stable electronic configuration. Thus `Mn^(3+)` easily reduced to `Mn^(2+)` and acts as oxidant.