The ions $\mathrm{O}^{2-}, \mathrm{F}^{-}, \mathrm{Na}^{+}, \mathrm{Mg}^{2+}$ and $\mathrm{Al}^{3+}$ are isoelectronic. Their ionic radii show:

• A

  a significant increase from $\mathrm{O}^{2-}$ to $\mathrm{Al}^{3+}$
• Correct answer
  B

  a significant decrease from $\mathrm{O}^{2-}$ to $\mathrm{Al}^{3+}$
• C

  an increase from $\mathrm{O}^{2-}$ to $\mathrm{F}^{-}$and then decrease from $\mathrm{Na}^{+}$to $\mathrm{Al}^{3+}$
• D

  a decrease from $\mathrm{O}^{2-}$ to $\mathrm{F}^{-}$and then increase from $\mathrm{Na}^{+}$to $\mathrm{Al}^{3+}$

Hence, the correct answer is option: 'B'.