Which one of the following species does not exist under normal conditions?

According to Molecular Orbital (MO) Theory, a molecule is unstable and does not exist if its bond order is zero.${\text{Be}}_2$ has 8 electrons. Configuration: $\sigma 1s^2,{\sigma }^{\ast }1s^2,\sigma 2s^2,{\sigma }^{\ast }2s^2$.Bond Order $=\frac{1}{2}(\text{bonding electrons}-\text{antibonding electrons})=\frac{1}{2}(4-4)=0$.Since the bond order is zero, ${\text{Be}}_2$ does not exist.