JEE Main 2016 — d and f Block Elements Question with Solution

In $\mathrm{M}\mathrm{n}{\mathrm{O}}_4^{2-}$, manganese is in $+6$ oxidation state, which is having the highest stability.
${\text{MnO}}_4^{-2}\Rightarrow \mathrm{x}+4\mathrm{ }\left(-2\right)=-2$
$\mathrm{x}=6$
${\mathrm{Cu}}^{+}\Rightarrow \mathrm{x}=+1$  It will disproportionate in aqueous solution.
${\mathrm{Cr}}^{+2}\Rightarrow \mathrm{x}=+2$ It will also disproportionate in aqueous solution.
${\mathrm{MnO}}_4^{-3}\Rightarrow \mathrm{x}+4\left(-2\right)=-3$ Mn is not stable in +5 oxidation state.
$\mathrm{x}=5$
Higher the oxidation state, more is the stability of the species.