In which of the following octahedral comple-xes of cobalt (atomic number = 27) will the magnitude of ${∆}_0$be the highest?

Δ_0​ depends upon ligands because they cause large crystal field splitting depending upon their position in spectrochemical series i.e CN⁻  > NH₃​ > H_2​O > C₂​O₄^2−​
Greater the donating nature of ligand greater will be the ligand strength and therefore higher will the CFSE in octahedral complex