Which of the following ions is the most stable?

The most stable complex among the following is

When $Co(II)$ chloride is dissolved in concentrated $HCl$,a blue solution is obtained. Upon dilution with water,the colour changes to pink because:

$Ag^{+} + NH_3 \rightleftharpoons [Ag(NH_3)]^{+}$; $k_1 = 3.5 \times 10^{-3}$
$[Ag(NH_3)]^{+} + NH_3 \rightleftharpoons [Ag(NH_3)_2]^{+}$; $k_2 = 1.7 \times 10^{-3}$
Then the formation constant of $[Ag(NH_3)_2]^{+}$ is

Which of the following factors tends to increase the stability of metal ion complexes?

Which complex is formed when an aqueous solution of $NH_3$ is added to a $CuSO_4$ solution,resulting in a deep blue color?