(i) $\mathrm{Cr}^{2+}$, because its configuration changes from $d^4$ to $d^3$ and having a half-filled $t_{2g}$ 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.$2 \mathrm{Cu}^{+} \rightarrow \mathrm{Cu}^{2+}(\mathrm{aq}) + \mathrm{Cu}(\mathrm{s})$(iii) $\mathrm{Mn}^{3+}$, because its configuration changes from $\mathrm{Mn}^{3+}$ to $\mathrm{Mn}^{2+}$ results in the half filled $d^5$ configuration, which has extra stability.