b
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 \(B e^{2+}\) \(H^{+} \rightarrow\) no eletron \(N a^{+} \rightarrow 10 e^{-}\)

\(L i^{+} \rightarrow 2 e^{-}\)

\(M g^{2+} \rightarrow 10 e^{-}\)

Hence, \(B e^{2+}\) is isoelectronic with \(L i^{+}\)