The complex ion having minimum wavelength of absorption in the visible region is

Here $\left[\mathrm{C}\mathrm{o}\right(\mathrm{N}{\mathrm{H}}_3{)}_6{]}^{3+}$ have minimum wavelength of absorption in the visible region as for it splitting energy is more as it has all strong ligands.
${\mathrm{\Delta }}_0$ (splitting energy) $\propto \frac{1}{\mathrm{\lambda }\left(\mathrm{A}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}\right)}$
Basic strength of ligand C > N > O > X (halogen) donor.