1. We compare the octahedral crystal field splitting $\Delta_o$ for the given complexes: $[M(H_2O)_6]^{3+}$ where the ligand is the same in all cases: $H_2O$.

2. Since the ligand is the same, the main factor affecting $\Delta_o$ is the nature of the metal ion.

3. For a given ligand, crystal field splitting generally increases:

   • with increasing oxidation state,
   • and importantly, from $3d < 4d < 5d$ transition series.

4. Here all metal ions are in the same oxidation state $+3$, so we compare based on the transition series:

   • $Cr^{3+}$ is a $3d$ metal
   • $Fe^{3+}$ is also a $3d$ metal
   • $Mo^{3+}$ is a $4d$ metal
   • $Os^{3+}$ is a $5d$ metal

5. For the same ligand and same oxidation state: $\Delta_o(5d) > \Delta_o(4d) > \Delta_o(3d)$ Therefore, $[Os(H_2O)_6]^{3+}$ will have the highest crystal field splitting.

6. Checking options:

   • A: $[Cr(H_2O)_6]^{3+}$ — smaller than $5d$
   • B: $[Mo(H_2O)_6]^{3+}$ — larger than $3d$, but smaller than $5d$
   • C: $[Fe(H_2O)_6]^{3+}$ — $3d$, not maximum
   • D: $[Os(H_2O)_6]^{3+}$ — maximum

Hence, the correct answer is D.