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

Species that have the same number of electrons and same electronic confi juration are called isoelectronics.
$\begin{array}{r}{\mathrm{Be}}^{2+},2\mathrm{e}-1{\mathrm{s}}^2\\ {\mathrm{Li}}^{+},2\overline{\mathrm{e}}-1{\mathrm{s}}^2\end{array}$
(1) ${\mathrm{H}}^{+}-1{\mathrm{s}}^{\circ }$
(3) ${\mathrm{Na}}^{+}-\left[\mathrm{Ne}\right]-10e^{-}$
(4) ${\mathrm{Mg}}^{+2}-\left[\mathrm{Ne}\right]-10{\mathrm{e}}^{-}$
Number of electrons are different from Be⁺² .The above ions have different number of electrons and different electronic configuration from Be⁺². Therefore, not isoelectronics.