Explanation

(c)
$O_{2}^{2 +}$ has 14 electrons. Its electronic configuration is as
$O_{2}^{+}\ :\ \sigma 1s^{2}\ _{\sigma}^{*}1s^{2}\ ,\sigma 2s^{2}\ _{\sigma}^{*}2s^{2},\ \pi 2p_{y}^{2}\ \pi 2p_{z}^{2}\sigma 2p_{x}^{2}$
Bond order $= \frac{N_{b - \ }N_{a\ }}{2} = \frac{10 - 4}{2} = 3$