Which factor does decide the acidic or basic nature of the oxide of an element?

Which of the following species are isoelectronic in nature?
$\left(\mathrm{i}\right){\mathrm{Ca}}^{2+} \left(\mathrm{ii}\right)\mathrm{K} \left(\mathrm{iii}\right){\mathrm{Mg}}^{2+} \left(\mathrm{iv}\right){\mathrm{S}}^{2-} \left(\mathrm{v}\right) {\mathrm{Cl}}^{-}$