Correct Answer - C
The magnitude of `Delta _(0) ` ( the crystal field splitting energy ) is decided by the nature of ligand strong ligand has highest `Delta _(0) ` . The increasing field strength is as
`l^(-) lt Br^(-) lt F^(-) lt OH^(-) lt C_(2)O_(4)^(2-) lt NH_(3)lt en lt NO_(2)^(-) lt CN^(-)`
The `CN^(-)` is the strongest ligand among the given , hence the magnitude of `Delta _(0)` will be maximum in `[ Co(CN)_(6)]^(3-)`.