Explanation

(b)
$O_{2}(8 + 8 = 16) = \sigma 1s^{2},\sigma^{*}1s^{2},\sigma 2s^{2},\sigma^{*}2s^{2},\sigma 2p_{\mathcal{z}}^{2},\pi 2p_{x}^{2}$
$\approx \pi 2p_{y}^{2},\pi^{*}2p_{x}^{1} \approx \pi^{*}2p_{y}^{1}$
Bond order $= \frac{10 - 6}{2} = 2$
$O_{2}^{+}(8 + 8 - 1 = 15)$
Bond order$= \frac{10 - 5}{2} = 2.5$
$O_{2}^{-}(8 + 8 + 1 = 17),$
Bond order $= \frac{10 - 7}{2} = 1.5$
$O_{2}^{2 -}(8 + 8 + 2 = 18),$
Bond order $= \frac{10 - 8}{2} = 1$
Thus, bond order is maximum for $O_{2}^{+}$