$B{e}^{2+}$ is isoelectronic with which of the following ions ?

Isoelectronic species contain same number of electrons. $B{e}^{2+}$ contains 2 electrons .Among the given options, only $L{i}^{+}$ contains 2 electrons and therefore, it is isoelectronic with $B{e}^{2+}$.
$H^{+}\to$ no electron , $N{a}^{+}\to 10e^{-}$
$L{i}^{+}\to 2e^{-}$
Hence, $B{e}^{2+}$ is isoelectronic with $L{i}^{+}$