Which of the following triads have unequal size?

• A

  ${\mathrm{Na}}^{\oplus },{\mathrm{Mg}}^{2+},{\mathrm{Al}}^{3+}\text{ (isoelectronic) }$

• B

  ${\mathrm{Mn}}^{\oplus },{\mathrm{Fe}}^{2+},\mathrm{Cr}\text{ (isoelectronic) }$

• C

  ${\mathrm{F}}^{\ominus },\mathrm{Ne},{\mathrm{O}}^{2-}\text{ (isoelectronic) }$

• D

  $\mathrm{Fe},\mathrm{Co},\mathrm{Ni}$