In which of the following coordination entities the magnitude of $ \Delta\: o$ (C F S E in octahedral field ) will be maximum ?

Question

$\begin {array} {1 1} (1)\;[CO(C_2O_4)_3]^{3-} & \quad (2)\;[CO(H_2O)_6]^{3+} \\ (3)\;[CO(NH_3)_6]^{3+} & \quad (4)\;[Co(CN)_6]^{3-} \end {array}$

(At. No. Co.=27)