Explanation

(b)
The MO electronic configuration of
$O_{2}^{-}\ (8 + 8 + 1 = 17) = \ \sigma 1s^{2},\ _{\sigma}^{*}1s^{2}\ ,\sigma 2s^{2},\ _{\sigma}^{*}2s^{2},\sigma 2p_{z}^{2}\ ,\ \pi 2p_{x}^{2} \approx \pi 2p_{y,}^{2}\ _{\pi}^{*}2p_{x,}^{2} \approx \ _{\pi}^{*}2p_{y}^{1}$
Bond order = $\frac{N_{b - N_{a}}}{2}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{10 - 7}{2} = 1.5$