Isoelectronic species are atoms and ions that contain the same number of electrons. The context explicitly states that $O^{2-}$, $F^-$, $Na^+$, and $Mg^{2+}$ have 10 electrons, providing the answers for (i) and (iii). For (ii) and (iv), we use the provided Group 1 electronic configurations: K is [Ar]$4s^{1}$ (19 electrons), so $K^+$ has 18 electrons, matching Ar. Rb is [Kr]$5s^{1}$ (37 electrons), so $Rb^+$ has 36 electrons, matching Kr.