Which of the following statements are correct? (A) The M ^ 3 + / M...

The valence electronic configuration of ${Mn}^{3 +}$ is $3{d}^{4}$ and that of ${Mn}^{2 +}$ is stable $3{d}^{5}$.The valence electronic configuration of ${Fe}^{3 +}$ is $3{d}^{5}$ and that of ${Fe}^{2 +}$ is $3{d}^{6}$.

The reduction electrode potential of ${Mn}^{3 +} / {Mn}^{2 +}$ is $+ 1 . 57V$ while that of ${Fe}^{3 +} / {Fe}^{2 +}$ is $+ 0 . 77V$, hence A is wrong. Higher oxidation state of smaller $d$-block elements is stabilised (or say form compounds) with smaller anion oxide that can be explained by stearic reason hence $B$ is correct.

The oxidation electrode potential of ${Cr}^{2 +} / {Cr}^{3 +}$ is $+ 0 . 41V$ hence, it can reduce ${H}^{+}$ and so liberate ${H}_{2}$.

The unpaired electrons in ${V}^{2 +}$ are $3$ hence, the magnetic moment of ${V}^{2 +}$ will be lesser than $4 . 4BM$.

Hence, only $B$ and $C$ are correct.