Which of the following complex is octahedral, diamagnetic and the most stable?

The coordination compound ${\mathrm{K}}_3\left[\mathrm{Co}\right(\mathrm{CN}{)}_6]$contains a cobalt($\mathrm{III}$) ion, which has a ${\mathrm{d}}^6$ electronic configuration. The cyanide ligands in this complex are indeed strong field ligands, which means they will cause a large splitting of the $\mathrm{d}$orbitals. So here cobalt is in $+3$ state having $3{\mathrm{d}}^6$ configuration and also cyanide ligand which is a strong field ligand so, it is most stable and will have octahedral shape. The given compounds have the following nature:

(1) ${\mathrm{Na}}_3\left[{\mathrm{CoCl}}_6\right]$ – Paramagnetic

(2) $\left[\mathrm{Ni}{\left({\mathrm{NH}}_3\right)}_6\right]{\mathrm{Cl}}_2$ – Paramagnetic

(3) ${\mathrm{K}}_3\left[\mathrm{Co}(\mathrm{CN}{)}_6\right]$– Diamagnetic

(4) $\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]{\mathrm{Cl}}_2$ – Paramagnetic