#### Answer

C

#### Work Step by Step

$O_2$ 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:
$[core]\ (\sigma_{2s})^2\ (\sigma_{2s}^*)^2\ (\sigma_{2p})^2\ (\pi_{2p})^4\ (\pi_{2p}^*)^0\ (\sigma_{2p}^*)^0$