Which of the following ions forms the most stable complex compound?

$5.33 \text{ g}$ of $CrCl_3 \cdot 6H_2O$, which is a $1:3$ electrolyte, is dissolved in water and passed through a cation exchanger. The chloride ions in the eluted solution, on treatment with $AgNO_3$, result in $8.61 \text{ g}$ of $AgCl$. The ratio of moles of complex reacted and moles of $AgCl$ formed is . . . . . . $\times 10^{-2}$. (Nearest integer) [Molar mass in $\text{g mol}^{-1}$: $Cr = 52, Ag = 108, Cl = 35.5, H = 1, O = 16$]

On adding an excess of $NH_3$ solution to $CuSO_4$ solution,the dark blue colour is due to the formation of which complex?

The purple color of $KMnO_4$ is due to which of the following?

Cold ferrous sulphate solution on absorption of $NO$ develops brown colour due to the formation of

Given the following reactions:
$Ag^{+} + NH_3 \rightleftharpoons [Ag(NH_3)]^+ ; K_1 = 1.6 \times 10^3$
$[Ag(NH_3)]^+ + NH_3 \rightleftharpoons [Ag(NH_3)_2]^+ ; K_2 = 6.8 \times 10^3$
What is the formation constant for $[Ag(NH_3)_2]^+$?