Which of the following complex is octahedral, diamagnetic and the most stable?
\( [\mathrm{Co}(\mathrm{H}_2\mathrm{O})_6]\mathrm{Cl}_2 \)
\( [\mathrm{Ni}(\mathrm{NH}_3)_6]\mathrm{Cl}_2 \)
\( \mathrm{K}_3[\mathrm{Co}(\mathrm{CN})_6] \)
\( \mathrm{Na}_3[\mathrm{CoCl}_6] \)