Iso-electronic ions are ions that have the same number of electrons, and iso-structural ions have the same structure.
- \( \text{CO}_3^{2-} \) and \( \text{NO}_3^{-} \) are not iso-structural, as their bond angles and geometries are different.
- \( \text{SO}_3^{2-} \) and \( \text{NO}_3^{-} \) are also not iso-structural.
- \( \text{ClO}_3^{-} \) and \( \text{CO}_3^{2-} \) are iso-electronic and iso-structural. Both have 24 electrons and adopt trigonal planar geometry.
Therefore, the correct answer is option (C).