Which aqueous solution absorbs nitric oxide in an appropriate amount?

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

Which of the following is the most stable complex?

What is the color of a cobalt chloride solution?

Which of the following ligands does not show synergic bonding with a central metal atom?

An aqueous solution of $FeSO_4$ gives tests for both $Fe^{2+}$ and $SO_4^{2-}$ ions,but after the addition of an excess of $KCN$,the solution ceases to give a test for $Fe^{2+}$. This is due to the formation of: