Transition metal complex with highest value of crystal field splitting Δ 0 will be

${\mathrm{H}}_2\mathrm{O}$ is a weak field ligand. Crystal field stabilisation energy is maximum for ${\mathrm{d}}^3$ and ${\mathrm{d}}^8$ cases with weak field ligands among the 3d-series elements.But $\mathrm{CFSE}$ increases from 3d to 5d series elements due to the increase in nuclear charge. Hence, ${\left[\mathrm{Os}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ complex case, the $\mathrm{CFSE}$ is maximum.