Which of the following complex will show largest splitting of d-orbitals?

Some ligands are able to produce strong fields in which case, the splitting will be large whereas others produce weak fields and consequently result in a small splitting of d orbitals.

In general, ligands can be arranged in a series called spectrochemical series according to which maximum splitting is done by the strongest ligand which is cyanide (${\mathrm{CN}}^{-}$) ion in this case.