Identify the species having one π-bond and maximum number of canonical forms from the following:

🧠 "One π-bond and maximum canonical forms" — the π-bond must be delocalised across the structure. More resonance structures = more canonical forms. Count canonical forms for each.

• $\ce{O2}$: 1 π-bond, essentially 1–2 canonical forms ✗ (fewest)
• $\ce{SO2}$: 1 delocalised π-bond, 2 canonical structures (resonance with each S=O) ✗
• $\ce{SO3}$: 3 canonical structures (resonance across 3 S=O bonds), but has 3 π-bonds (or 1 delocalised over 3 positions) — multiple π-bonds ✗
• $\ce{CO3^{2-}}$: C=O double bond delocalised over 3 equivalent positions → 3 canonical structures, and there is effectively 1 π-bond (delocalised) → ✓ maximum canonical forms with one π-bond

⚡ $\ce{CO3^{2-}}$, $\ce{NO3-}$, and $\ce{SO3}$ are all trigonal planar with 3-fold resonance, but the question restricts to "one π-bond" — $\ce{CO3^{2-}}$ best fits: 1 delocalised π-bond over 3 canonical forms.

$\boxed{\text{Answer: (d) } \ce{CO3^{2-}}}$