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

Question

$\mathrm{Na}_{3}\left[\mathrm{CoCl}_{6}\right]$

$\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{2}$

$\mathrm{K}_{3}\left[\mathrm{Co}(\mathrm{CN})_{6}\right]$

$\left[\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}$

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