CFSE (in the octahedral field) will be maximum in: (Atomic number Co = 27)

Crystal Field Stabilization Energy (CFSE) depends on the magnitude of the crystal field splitting energy ($\Delta_o$). The value of $\Delta_o$ is directly determined by the field strength of the ligands coordinating to the central metal ion.

1. Oxidation State: In all the given complexes, Cobalt is in the +3 oxidation state ($3d^6$ configuration).
2. Spectrochemical Series: According to the spectrochemical series found in the NCERT text, the order of increasing field strength of the ligands involved is: $H_2O < C_2O_4^{2-} < NH_3 < CN^-$
3. Conclusion: The cyanide ion ($CN^-$) is the strongest field ligand among the options. It causes the largest splitting of the d-orbitals (maximum $\Delta_o$). Since CFSE is directly proportional to $\Delta_o$ (specifically for a low-spin $d^6$ complex, CFSE = $-2.4\Delta_o$), the complex with the strongest ligand will exhibit the maximum crystal field stabilization energy .