(C) Isoelectronic species are those that have the same number of electrons.$Mg$ has an atomic number of $12$,so $Mg^{2+}$ has $12 - 2 = 10$ electrons.

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(C) Isoelectronic species are those that have the same number of electrons.$Mg$ has an atomic number of $12$,so $Mg^{2+}$ has $12 - 2 = 10$ electrons.$Na$ has an atomic number of $11$,so $Na^{+}$ has $11 - 1 = 10$ electrons.Since both $Mg^{2+}$ and $Na^{+}$ have $10$ electrons,they are isoelectronic.