Which among the following is the strongest ligand?

On treatment of $100 \ mL$ of $0.1 \ M$ solution of $CoCl_3 \cdot 6H_2O$ with excess $AgNO_3$,$1.2 \times 10^{22}$ ions are precipitated. The complex is:

The $[Fe(CN)_6]^{3-}$ complex ion

Select the most stable complex.

The compound $ZnFe_2O_4$ is:

Which among the following cations forms a least stable complex if the ligand is same?