For Na + ,   Mg 2 + ,   F - and O 2 - , the correct order of increasing ionic radii is:

Isoelectric series:

${\mathrm{Mg}}^{2+}<{\mathrm{Na}}^{+}<{\mathrm{F}}^{-}<{\mathrm{O}}^{2-}$

All have $10$ electrons. In the isoelectronic series, the proton/electron ratio decreases, so the size decrease because ${\mathrm{Z}}_{\mathrm{eff}}$ increases on decrease proton/electron ratio.