In which of the following coordination entites the magnitude of ${\Delta }_0$ (CFSE in octehedral field) will be maximum.
$\left(At.No.Co=27\right)$

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 ${\left[Co{\left(CN\right)}_6\right]}^{3-}$ magnitude of CFSE i.e. ${\Delta }_0$ is maximum.