The correct option is A CN−
Ligands can be arranged in a series of increasing field strength known as spectrochemical series.
Spectrochemical series:
I−<Br−<SCN−<Cl−<S2−<F−<−OH<C2O2−4<H2O<NCS−<EDTA4−<NH3<en (Ethylenediamine) <NO−2<CN−<CO
Hence, CN− is the strong field ligand.