The increasing order of field strength of ligands is
 

Cyanide is the strongest ligand. ${\mathrm{NH}}_3$ is a weak ligand.  

Carbonyl ligand is strong as it contains empty orbitals which bring in splitting in the d orbitals of the metal atom. 

Using the idea of a spectrochemical series, ligands are placed in a crystal field strength-increasing sequence is ${\mathrm{NH}}_3<{\mathrm{H}}_2\mathrm{O}<\mathrm{Cl}<\mathrm{CO}<\mathrm{CN}$
Hence the correct answer is ${\mathrm{NH}}_3<{\mathrm{H}}_2\mathrm{O}<\mathrm{Cl}<\mathrm{CO}<\mathrm{CN}$