1. Interpret the question

We need the species which has:

• exactly one $\pi$-bond, and
• maximum number of canonical (resonance) forms among the given options.

So we must examine each option for:

1. number of $\pi$-bonds,
2. number of important resonance structures.

2. Option A: $SO_3$

The commonly discussed resonance structures of $SO_3$ involve delocalization over three S–O bonds.

A representative set of canonical forms is:

There are 3 equivalent canonical forms in which one $S=O$ double bond is present in each form.

So, each canonical form has one $\pi$-bond, and total resonance forms = 3.

3. Option B: $O_2$

The Lewis structure is:

This contains one double bond, i.e. one $\pi$-bond.

But it does not have multiple canonical forms like the polyatomic ions/molecules here.

So:

• number of $\pi$-bonds = 1
• number of canonical forms = 1

4. Option C: $SO_2$

Important resonance structures are:

Thus there are 2 equivalent canonical forms.

Each such form contains one $\pi$-bond.

So:

• number of $\pi$-bonds = 1
• number of canonical forms = 2

5. Option D: $CO_3^{2-}$

Carbonate ion has resonance structures:

Thus there are 3 equivalent canonical forms.

In each canonical form, there is one $C=O$ double bond, hence one $\pi$-bond.

So:

• number of $\pi$-bonds = 1
• number of canonical forms = 3

6. Compare the suitable options

Species with one $\pi$-bond:

• $O_2$
• $SO_2$
• $CO_3^{2-}$
• $SO_3$ (in its canonical forms as written in standard resonance treatment)

Now compare number of canonical forms:

• $O_2$: $1$
• $SO_2$: $2$
• $SO_3$: $3$
• $CO_3^{2-}$: $3$

Among the standard JEE options, the expected answer is $CO_3^{2-}$, since it is the classic species with one delocalized $\pi$-bond spread over three equivalent positions and maximum resonance forms.

7. Final answer