$CuSO_4$ reacts with $KCN$ solution and forms

$A$ 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 $(Ag = 108 \ u)$:

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

Amongst the following,the most stable complex is:
$(i)$ $[Fe(H_2O)_6]^{3+}$
$(ii)$ $[Fe(NH_3)_6]^{3+}$
$(iii)$ $[Fe(C_2O_4)_3]^{3-}$
$(iv)$ $[FeCl_6]^{3-}$

The brown ring test for $NO_2^-$ and $NO_3^-$ ions is due to the formation of which of the following complex ions?

In ${K_4}[Fe(CN)_6]$,$Fe$ is in the form of