Which of the following complex ions is most stable?

The stability of a complex ion can be decided with effective atomic number rule.

$\mathrm{EAN} = \mathrm{Atomic} \mathrm{number} \mathrm{of} \mathrm{metal}-\mathrm{O}.\mathrm{N}. \mathrm{of} \mathrm{metal}+2\times \mathrm{C}.\mathrm{N}.$

Oxidation number of cobalt in given complex: $$\begin{gathered} \mathrm{X}-6 =-3 \\ \mathrm{X}= +3 \end{gathered}$$

So,$\mathrm{EAN} =27-3+2\times 6=36$. EAN of this complex is equal to the atomic number next noble gas,so it follows EAN rule. So, it is most stable among given complexes.