Which of the following has fractional bond order?

A simplified application of MO theory to the hypothetical 'molecule' $\mathrm{OF}$ would give its bond order as:

The following molecules/species have been arranged in the order of their increasing bond orders. Identify the correct order.

($\mathrm{I}$) ${\mathrm{O}}_2$;

($\mathrm{II}$) ${\mathrm{O}}_2^{-}$;

($\mathrm{III}$) ${\mathrm{O}}_2^{2-}$;

($\mathrm{IV}$) ${\mathrm{O}}_2^{+}$;