Which among the following cations will form the lowest stability complex if the ligand remains the same?

In the change $[Cu(H_2O)_6]^{2+} \xrightarrow{HCl} [CuCl(H_2O)_5]^+$,the colour changes from

$A$ solution containing $2.675 \ g$ of $CoCl_3 \cdot 6NH_3$ (molar mass $= 267.5 \ g \ mol^{-1}$) is passed through a cation exchanger. The chloride ions obtained in the solution are treated with excess $AgNO_3$ to give $4.78 \ g$ of $AgCl$ (molar mass $= 143.5 \ g \ mol^{-1}$). The formula of the complex is $(Ag = 108 \ u)$:

The hydrated $Cu^{2+}$ ion is:

$CuSO_4$ solution reacts with excess $KCN$ to give

Choose the incorrect stability order.