Which of the following diatomic molecules is not observed?

Diatomic molecules are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen or oxygen, then it is said to be homonuclear

${\mathrm{Be}}_2$ is not observed as the number of electrons in the bonding orbitals is equal to the number of electrons in the antibonding orbitals (i.e. bond order of ${\mathrm{Be}}_2$ is zero).