In which of the following octahedral complexes of $\text{Co}$ (at. no. 27) will the magnitude of ${\Delta }_o$ be the highest?

The magnitude of crystal field splitting (${\Delta }_o$) depends on the strength of the ligand according to the spectrochemical series:${\text{I}}^{-}<{\text{Br}}^{-}<{\text{S}}^{2-}<{\text{Cl}}^{-}<{\text{F}}^{-}<{\text{OH}}^{-}<{\text{C}}_2{\text{O}}_4^{2-}<{\text{H}}_2\text{O}<{\text{NH}}_3<\text{en}<{\text{NO}}_2^{-}<{\text{CN}}^{-}$ Cyanide (${\text{CN}}^{-}$) is a very strong field ligand. Stronger ligands cause greater splitting of the $d$ -orbitals, resulting in a higher ${\Delta }_o$.