Final answer:
Na⁺ is not isoelectronic with the other options.
Step-by-step explanation:
The element that is not isoelectronic with the others is c. Na⁺.
Isoelectronic species have the same number of electrons. In this case, we need to compare the electron configurations of the given ions/atoms.
F⁻ has 10 electrons, Ne has 10 electrons, Mg⁺ has 11 electrons, and Na⁺ has 10 electrons. Since Na⁺ has a different number of electrons compared to the other three, it is not isoelectronic with them.