a
The splitting (value of \(\Delta_{0}\) ) increases as the strength of ligands. The order of strength of ligands attached with \(Co ^{3+}\) ion is as follows :

\(CN ^{- } > NH _{3} > H _{2} O > Cl ^{-}\)

Wavelength absorbing light \(\alpha \frac{1}{\Delta_{0}}\)

Therefore, the correct order for the wavelength of absorption will be :

\(\left[ Co ( CN )_{6}\right]^{3-},\left[ Co \left( NH _{3}\right)_{6}\right]^{3+},\left[ Co \left( NH _{3}\right)_{5}\left( H _{2} O \right)\right]^{3+}\) \(\left[ Co \left( NH _{3}\right)_{5} Cl \right]^{+2}\)