Which complex compound is most stable?

The strongest ligand among the following is:

The correct order of molar conductivity of the following complex compounds in aqueous solution is:
$(I) K[Co(NH_3)_2(NO_2)_4]$
$(II) [Cr(NH_3)_3(NO_2)_3]$
$(III) [Cr(NH_3)_5NO_2]_3[Co(NO_2)_6]_2$
$(IV) Mg[Cr(NH_3)(NO_2)_5]$

When concentrated $HCl$ is added to an aqueous solution of $CoCl_2$,its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction?

Which one of the following will give a white precipitate with $AgNO_3$ in aqueous medium?

The compounds containing complex oxyanion $CrO_4^{2-}$ are intensely yellow coloured because