Spin correlation and its exchange energy gives an electronic configuration a special stability which is greatest for half Filled electronic configurations. $\mathrm{Mn}^{2+}\left(\mathrm{d}^{5}\right)$ gets stabilisation due to half-filled configuration.

In $Fe^{2+}\left(d^{6}\right)$ the placing of one extra electron in a subshell destabilises. Placing of $2$ electrons in $\mathrm{Co}^{2+}\left(\mathrm{d}^{7}\right)$ destabilises it more. $\mathrm{Cr}^{2+}\left(\mathrm{d}^{4}\right)$ has one vacant subshell. Fe $^{2+}$ gets more stabilisation compared to $\mathrm{Cr}^{2+}$ through exchange energy. So the order is as follows:

$\mathrm{Mn} > \mathrm{Fe} > \mathrm{Cr} > \mathrm{Co}$