1. Given complexes

All the complexes are octahedral aqua complexes of the type $[M(H_2O)_6]^{3+}$.

So, the ligand is the same in all cases: $H_2O$. Hence, the crystal field splitting energy $\Delta_o$ depends mainly on the metal ion.

2. Rule for comparing $\Delta_o$

For a given ligand and same oxidation state, in the first transition series:

• $\Delta_o$ generally increases from left to right across the period.
• It also increases with increasing effective nuclear charge and decreasing ionic radius.

Thus, among $\mathrm{Ti}^{3+}, \mathrm{Cr}^{3+}, \mathrm{Mn}^{3+}, \mathrm{Fe}^{3+}$, we compare their positions in the 3d series.

3. Identify the metal ions

• $\mathrm{Ti}^{3+} : 3d^1$
• $\mathrm{Cr}^{3+} : 3d^3$
• $\mathrm{Mn}^{3+} : 3d^4$
• $\mathrm{Fe}^{3+} : 3d^5$

4. Special stability of $d^3$ octahedral complexes

For octahedral complexes, $d^3$ configuration gives:

$t_{2g}^3 e_g^0$

This has strong stabilization with no electrons in antibonding $e_g$ orbitals, and such ions typically show relatively large crystal field splitting among comparable aqua complexes.

Also, standard trends for hexaaqua $M^{3+}$ ions show:

$[\mathrm{Cr}(H_2O)_6]^{3+}$

has the highest $\Delta_o$ among the given options.

5. Check options

• A: $[\mathrm{Fe}(H_2O)_6]^{3+}$ — not highest
• B: $[\mathrm{Mn}(H_2O)_6]^{3+}$ — not highest
• C: $[\mathrm{Cr}(H_2O)_6]^{3+}$ — highest $\Delta_o$
• D: $[\mathrm{Ti}(H_2O)_6]^{3+}$ — not highest

6. Final answer

Therefore, the complex with highest magnitude of crystal field splitting energy is:

$\boxed{[\mathrm{Cr}(H_2O)_6]^{3+}}$

So, Option C is correct.