${\text{Mg}}^{2+}$ is isoelectronic with

Isoelectronic species have the same number of electrons.
* The atomic number of Magnesium ($\text{Mg}$) is 12. Therefore, ${\text{Mg}}^{2+}$ has $12-2=10$ electrons.
* Let's check the options:
* ${\text{Ca}}^{2+}$: Atomic number of $\text{Ca}=20$. Number of electrons in ${\text{Ca}}^{2+}=20-2=18$.
* ${\text{Na}}^{+}$: Atomic number of $\text{Na}=11$. Number of electrons in ${\text{Na}}^{+}=11-1=10$.
* ${\text{Zn}}^{2+}$: Atomic number of $\text{Zn}=30$. Number of electrons in ${\text{Zn}}^{2+}=30-2=28$.
* ${\text{Cu}}^{+}$: Atomic number of $\text{Cu}=29$. Number of electrons in ${\text{Cu}}^{+}=29-1=28$.

Thus, ${\text{Mg}}^{2+}$ and ${\text{Na}}^{+}$ both have 10 electrons and are isoelectronic.