From the following pairs of ions which one is not an iso-electronic pair?

Isoelectronic species have the same number of electrons.${\text{Mn}}^{2+}$ ($Z=25-2=23$) and ${\text{Fe}}^{3+}$ ($Z=26-3=23$). Isoelectronic.${\text{Fe}}^{2+}$ ($Z=26-2=24$) and ${\text{Mn}}^{2+}$ ($Z=25-2=23$). NOT isoelectronic.${\text{O}}^{2-}$ ($8+2=10$) and ${\text{F}}^{-}$ ($9+1=10$). Isoelectronic.${\text{Na}}^{+}$ ($11-1=10$) and ${\text{Mg}}^{2+}$ ($12-2=10$). Isoelectronic.