Isoelectronic species are defined as atoms, ions, or molecules that possess the same total number of electrons . To identify the isoelectronic pair, we calculate the total electron count for the species in the correct option using their atomic numbers ($Z$) and charges:

1. $CN^{-}$ (Cyanide ion): Carbon ($Z=6$) + Nitrogen ($Z=7$) + 1 (for the negative charge) = 14 electrons .
2. $CO$ (Carbon monoxide): Carbon ($Z=6$) + Oxygen ($Z=8$) = 14 electrons .

Since both $CN^{-}$ and $CO$ have exactly 14 electrons, they are isoelectronic . Other pairs, such as $CO_{2}$ (22 electrons) and $NO_{2}$ (23 electrons), do not have the same number of electrons and are therefore not isoelectronic .