(i) ${\mathrm{Cr}}^{2+}$, because its configuration changes from ${\mathit{d}}^4$ to ${\mathit{d}}^3$ and having a half-filled ${\mathit{t}}_{2\mathit{g}}$ level.
(ii) ${\mathrm{Cu}}^{+}$in an aqueous medium energy is required to remove one electron from ${\mathrm{Cu}}^{+}$to ${\mathrm{Cu}}^{2+}$, high hydration energy of ${\mathrm{Cu}}^{2+}$ compensates for it. Therefore ${\mathrm{Cu}}^{+}$ion in an aqueous solution is unstable.
$2{\mathrm{Cu}}^{+}\to {\mathrm{Cu}}^{2+}\left(\mathrm{aq}\right)+\mathrm{Cu}\left(\mathrm{}\mathit{s}\right)$
(iii) ${\mathrm{Mn}}^{3+}$, because its configuration changes from ${\mathrm{Mn}}^{3+}$ to ${\mathrm{Mn}}^{2+}$ results in the half filled ${\mathit{d}}^5$ configuration, which has extra stability.