Which one of the following high-spin complexes has the largest CSFE (Crystal field stabilization energy)?

Since  ${\mathrm{Cr}}^{9+}$ in ${\left[{\mathrm{Cr}}_{\mathrm{r}}\left({\mathrm{H}}_2{\mathrm{O}}_6\right)\right]}^{2+}$ has the higher oxidation state it has largest CSFE.

${\mathrm{Cr}}^{9+}\Rightarrow d^3,t_{2g}^{1,1,1}e{g}^{0,0},\mathrm{CFSE}=-1.2{\mathrm{\Delta }}_0$

${\mathrm{Mn}}^{3+}\Rightarrow d^4,t_{9g}^{1,1,1}{e}^{1,0}$

$\begin{array}{l}\mathrm{CFSE}=-1.2{\mathrm{\Delta }}_0+0.6{\mathrm{\Delta }}_0\\ =-0.6{\mathrm{\Delta }}_0\end{array}$

Hence correct option is D