a
\(\mathrm{Cr}^{2+}\) is reducing as it configuration changes from \(\mathrm{d}^4\) to \(\mathrm{d}^3\) due to formation of \(\mathrm{Cr}^{3+}\), which has half filled \(\mathrm{t}_{2 \mathrm{~g}}\) level, on other hand, the change \(\mathrm{Mn}^{3+}\) to \(\mathrm{Mn}^{2+}\) result half filled \(d^3\) configuration which has extra stability.