Paramagnetic species among the following is:

According to Molecular Orbital Theory, species with unpaired electrons are paramagnetic. Let us calculate the total number of electrons for each species:

• $\text{CO}$: $6 \text{ (C)} + 8 \text{ (O)} = 14$ electrons. It has no unpaired electrons and is diamagnetic.
• $\text{O}_2^-$: $16 \text{ (O}_2\text{)} + 1 = 17$ electrons. Its electronic configuration is $\sigma 1s^2 \sigma^* 1s^2 \sigma 2s^2 \sigma^* 2s^2 \sigma 2p_z^2 (\pi 2p_x^2 = \pi 2p_y^2) (\pi^* 2p_x^2 = \pi^* 2p_y^1)$. It has 1 unpaired electron in the antibonding $\pi^*$ orbital, making it paramagnetic.
• $\text{CN}^-$: $6 \text{ (C)} + 7 \text{ (N)} + 1 = 14$ electrons. It has no unpaired electrons and is diamagnetic.
• $\text{NO}^+$: $7 \text{ (N)} + 8 \text{ (O)} - 1 = 14$ electrons. It has no unpaired electrons and is diamagnetic. Therefore, $\text{O}_2^-$ is the only paramagnetic species among the given options.