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

• A

  $\mathrm{SO}_3$
• B

  $\mathrm{O}_2$
• C

  $\mathrm{SO}_2$
• Correct answer
  D

  $\mathrm{CO}_3^{2-}$

Hence, the correct answer is option: 'D'.