Which of the following species gains stability (its bond order increases) when it accepts one additional electron?

In $\mathrm{C}{}_2$ the added electron goes into the bonding $\sigma 2p_z$ orbital, raising bond order from 2 to 2.5. In $\mathrm{N}{}_2$, $\mathrm{O}{}_2$ and $\mathrm{F}{}_2$ the added electron enters an antibonding orbital (${\pi }^{\ast }2p$ or ${\sigma }^{\ast }2p$), lowering the bond order.