Wavelength \(\propto 1/\Delta_o\) (crystal field splitting). Stronger field ligands → larger \(\Delta_o\) → shorter wavelength absorbed. Ligand field strength: CN\(^-\) \(>\) NH\(_3\) \(>\) H\(_2\)O. Also Co\(^{3+}\) has larger \(\Delta_o\) than Cu\(^{2+}\) or Ti\(^{3+}\). [Co(CN)\(_6\)]\(^{3-}\): highest \(\Delta_o\) → shortest \(\lambda\). [Co(NH\(_3\))\(_6\)]\(^{3+}\) \(>\) [Ti(H\(_2\)O)\(_6\)]\(^{3+}\) \(>\) [Cu(H\(_2\)O)\(_4\)]\(^{2+}\). Order of \(\lambda\): C \(>\) D \(>\) A \(>\) B, so increasing: B \(<\) A \(<\) D \(<\) C = option (2) C\(<\)A\(<\)D\(<\)B is wrong. Correct: B\(<\)A\(<\)D\(<\)C = option (4).