Isoelectronic species are defined as atoms, ions, or molecules that possess the same total number of electrons . To identify the correct set, the total electron count for each species in Option A is calculated using their atomic numbers ($Z$) and ionic charges :

1. $CO$: Carbon ($Z=6$) + Oxygen ($Z=8$) = $14$ electrons .
2. $CN^{-}$: Carbon ($6$) + Nitrogen ($7$) + $1$ (for the negative charge) = $14$ electrons .
3. $NO^{+}$: Nitrogen ($7$) + Oxygen ($8$) - $1$ (for the positive charge) = $14$ electrons .
4. $C_{2}^{2-}$: Two Carbon atoms ($2 \times 6$) + $2$ (for the $2-$ charge) = $14$ electrons.

Since every species in this group has exactly $14$ electrons, they are isoelectronic . Other options are incorrect because they include species with different electron counts, such as $NO$ (15 electrons) or $CN$ (13 electrons) .