In which of the following octahedral complexes of CO(at.no=27),will be magnitude of $\Delta_0$ be the highest

Question

$(a)\;[CO(CN)_6]^{3-}\qquad(b)\;[CO(C_2O_4)_3]^{3-}\qquad(c)\;[CO(H_2O)_6]^{3+}\qquad(d)\;[CO(NH_3)_6]^{3+}$