${\text{Be}}^{2+}$ is isoelectronic with which of the following ions?

Isoelectronic species have the same number of electrons.$\text{Be}$ (At. No. 4) has 4 electrons. ${\text{Be}}^{2+}$ has $4-2=2$ electrons.${\text{H}}^{+}$: $1-1=0$ electrons.${\text{Li}}^{+}$ (At. No. 3): $3-1=2$ electrons.${\text{Na}}^{+}$: $11-1=10$ electrons.${\text{Mg}}^{2+}$: $12-2=10$ electrons.Thus, ${\text{Be}}^{2+}$ and ${\text{Li}}^{+}$ are isoelectronic.