Explanation

(a)
The correct option is $O_{2}^{2 -}$. This species has 18 electrons, which are filled in such a way that all molecular orbitals are fully filled, so diamagnetic.
$\sigma 1s^{2}\ _{\sigma}^{*}1s^{2}\ ,\sigma 2s^{2}\ _{\sigma}^{*}2s^{2}\sigma 2p_{z}^{2},\pi 2p_{x}^{2} = \pi 2p_{y}^{2},\ _{\pi}^{*}2p_{x}^{2} = \ _{\pi}^{*}2p_{y}^{2}$