Paramagnetism is exhibited by substances that have unpaired electrons. To determine if a species is paramagnetic or diamagnetic, we can analyze its molecular orbital (MO) configuration or simply count the total number of electrons.

Species with an odd number of total electrons are always paramagnetic. Species with an even number of total electrons can be either paramagnetic or diamagnetic. For even electron species, we need to check their MO configuration. If there are unpaired electrons in the MO configuration, the species is paramagnetic; otherwise, it is diamagnetic.

Let's count the total number of electrons for each species:

• A) NO: Nitrogen (7 electrons) + Oxygen (8 electrons) = 15 electrons. Since 15 is an odd number, NO has unpaired electrons and is paramagnetic.
• B) CO: Carbon (6 electrons) + Oxygen (8 electrons) = 14 electrons. Since 14 is an even number, we need to check its MO configuration. The MO configuration for CO is $({\sigma }_{1s}{)}^2({\sigma }_{1s}^{\ast }{)}^2({\sigma }_{2s}{)}^2({\sigma }_{2s}^{\ast }{)}^2({\pi }_{2p}{)}^4({\sigma }_{2p}{)}^2$. All electrons are paired. Therefore, CO is diamagnetic (not paramagnetic).
• C) O${}_2$: Oxygen (8 electrons) + Oxygen (8 electrons) = 16 electrons. Since 16 is an even number, we check its MO configuration. The MO configuration for O${}_2$ is $({\sigma }_{1s}{)}^2({\sigma }_{1s}^{\ast }{)}^2({\sigma }_{2s}{)}^2({\sigma }_{2s}^{\ast }{)}^2({\sigma }_{2p}{)}^2({\pi }_{2p}{)}^4({\pi }_{2p}^{\ast }{)}^2$. The two electrons in the ${\pi }_{2p}^{\ast }$ antibonding orbitals are unpaired (one in each orbital). Therefore, O${}_2$ is paramagnetic.
• D) B${}_2$: Boron (5 electrons) + Boron (5 electrons) = 10 electrons. Since 10 is an even number, we check its MO configuration. The MO configuration for B${}_2$ is $({\sigma }_{1s}{)}^2({\sigma }_{1s}^{\ast }{)}^2({\sigma }_{2s}{)}^2({\sigma }_{2s}^{\ast }{)}^2({\pi }_{2p}{)}^2$. The two electrons in the ${\pi }_{2p}$ bonding orbitals are unpaired (one in each orbital, according to Hund's rule). Therefore, B${}_2$ is paramagnetic.

Thus, CO is the only species that is not paramagnetic (it is diamagnetic).

Correct Answer: (B)