Among the following group which represents the collection of isoelectronic species ?

Species having equal number of electrons ae known as isoelectronic species.
Number of electrons ,
In CO =6+8=14
In $N{O}^{+}$=7+8-1=14
In $C{N}^{-}$=6+7+1=14
In $C_2^{2-}$ =12+2=14
Hence, all have 14 electrons, so they are isoelectronic species.