Explanation

(d)
CO, $CN^{-}$ and $NO^{+}$ all the three species have 14 electrons.
According to MOT
$\ \sigma 1s^{2},\ _{\sigma}^{*}1s^{2}\ ,\sigma 2s^{2},\ _{\sigma}^{*}2s^{2}\ ,\ \sigma 2p_{x}^{2}(\pi 2p_{y}^{2} = \pi 2p_{z}^{2})$
Bond order $= \frac{10 - 4}{2} = 3$
Similarly bond order for $O_{2}^{+} = 2.5$