Using MO theory predict which of the following species has the shortest bond length?

Bond length is inversely proportional to Bond Order.${\text{O}}_2$: $\text{BO}=2.0$${\text{O}}_2^{2+}$: $\text{BO}=3.0$ (Remove 2 antibonding $e^{-}$)${\text{O}}_2^{+}$: $\text{BO}=2.5$${\text{O}}_2^{-}$: $\text{BO}=1.5$${\text{O}}_2^{2-}$: $\text{BO}=1.0$ Since ${\text{O}}_2^{2+}$ has the highest bond order, it has the shortest bond length.