The least stable -ion among the following is

$\begin{array}{l}\mathrm{Li}=1{\mathrm{s}}^{2}2{\mathrm{s}}^1,\mathrm{Be}=1{\mathrm{s}}^2,2{\mathrm{s}}^2,\mathrm{B}=1{\mathrm{s}}^2,2{\mathrm{s}}^2,2{\mathrm{p}}^1\\ \mathrm{C}=1{\mathrm{s}}^2,2{\mathrm{s}}^{2}2{\mathrm{p}}^2\end{array}$

Be is highly stable due to filled s-orbitals, therefore ${\mathrm{Be}}^{⊝}$ is highly unstable.