Chapter 9 Bonding and Molecular Structure: Orbital Hybridization and Molecular Orbitals - 9-3 Molecular Orbital Theory - Review & Check for Section 9-3 - Page 365: 3

C

$O_2$ has 12 valence electrons, $B_2$ has 6, $C_2$ has 10, and $N_2^+$ has 9. Only $C_2$ doesn't have unpaired electrons: $[core]\ (\sigma_{2s})^2\ (\sigma_{2s}^*)^2\ (\sigma_{2p})^2\ (\pi_{2p})^4\ (\pi_{2p}^*)^0\ (\sigma_{2p}^*)^0$