Among the following, the paramagnetic species is

According to molecular orbital theory to find the paramagnetic species first we should calculate the total number of electrons present in a given ion or molecule. If total number of electrons for a particular molecule or ion is even then the species has non paramagnetic nature. If the total number of electrons is odd then the species has paramagnetic character. Here we will find the total number of electrons for a given ions or molecules as follows:

$\mathrm{For} {\mathrm{CN}}^{-}\mathrm{total} \mathrm{no}. \mathrm{of} {\mathrm{e}}^{-} \mathrm{s}= 6+ 7 +1= 14 \left(\mathrm{even}\right) \to \mathrm{non} \mathrm{paramagnetic}$

$\mathrm{For} \mathrm{NO} \mathrm{total} \mathrm{no}. \mathrm{of} {\mathrm{e}}^{-}\mathrm{s}=7+8=15\left(\mathrm{odd}\right)\to \mathrm{paramagentic}$

$\mathrm{For} {\mathrm{O}}_2^{-2}\mathrm{total} \mathrm{no}. \mathrm{of} {\mathrm{e}}^{-}\mathrm{s}=2\left(8\right)+2=18\left(\mathrm{even}\right)\to \mathrm{Non} \mathrm{paramagentic}$

For CO total no.of ${\mathrm{e}}^{-}\mathrm{S}=6+8=14\left(\mathrm{even}\right)\to \mathrm{non} \mathrm{paramagentic}$