Which one of the following complex ions is diamagnetic in nature?

Total number of geometrical isomers possible for the complexes $\left[\mathrm{NiCl}_4\right]^{2-}$, $\left[\mathrm{CoCl}_2\left(\mathrm{NH}_3\right)_4\right]^{+}$, $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_3\left(\mathrm{NO}_2\right)_3\right]$ and $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5 \mathrm{Cl}\right]^{2+}$ is

Arrange the following complexes in the increasing order of their spin only magnetic moment (in B.M)

I. $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}$

II. $\left[\mathrm{MnCl}_4\right]^{2-}$

III. $\left[\mathrm{Mn}(\mathrm{CN})_6\right]^{4-}$

IV. $\left.\left[\mathrm{Cr}(\mathrm{NH})_3\right)_6\right]^{3+}$

When 100 mL of 0.2 M solution of $\mathrm{CoCl}_3 \cdot x \mathrm{NH}_3$ is treated with excess of $\mathrm{AgNO}_3$ solution, $3.6 \times 10^{22}$ ions are precipitated. The value of $x$ is $\left(N=6 \times 10^{23} \mathrm{~mol}^{-1}\right)$