(b)
Species $\ {\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ O}_{2}\ \ \ \ \ \ \ \ O_{2}^{+}\ \ \ \ \ O_{2}^{2 +}\ \ \ \ O_{2}^{2 -}\ \ \$
Bond Order 2 2.5 3 1
Hence, the increasing bond order is as follows :
$O_{2}^{2 -} < O_{2} < O_{2}^{+} < O_{2}^{2 +}$