All the given species ($O^{2-},F^{-},N{a}^{+},M{g}^{2+}$) are isoelectronic, meaning they all have 10 electrons.

• For isoelectronic species, the ionic radius is inversely proportional to the atomic number ($Z$) (nuclear charge).
• $O^{2-}$ ($Z=8$): 8 protons pull 10 electrons (Weakest pull $\to$ Largest size)
• $F^{-}$ ($Z=9$): 9 protons pull 10 electrons
• $N{a}^{+}$ ($Z=11$): 11 protons pull 10 electrons
• $M{g}^{2+}$ ($Z=12$): 12 protons pull 10 electrons (Strongest pull $\to$ Smallest size) Trend: $O^{2-}>F^{-}>N{a}^{+}>M{g}^{2+}$.