a
As in all the given complex the central metal atom is same and contains same number of d electrons, thus \(CFSE\) is decided by ligands. In case of strong field ligand, \(CFSE\) is maximum. \(C N^{-}\) is a strong field ligand, Hence, in \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-} \mathrm{CFSE}\) is maximum. The magnitude of \(\Delta_{o c t}\) will be maximum in \(\left[\mathrm{Co}(\mathrm{CN})_{6}\right]^{3-}\)