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

First, remember that in any covalent structure a double bond contains one σ-bond and one π-bond, while a single bond contains only a σ-bond. Hence, to know the number of $$\pi$$-bonds we simply count the number of double bonds present in the Lewis structure of each species; every double bond contributes exactly one $$\pi$$-bond.

We also recall that canonical (resonance) forms are different valid Lewis structures obtained by shifting only electrons (not nuclei). The greater the number of ways in which double bonds and lone pairs can be redistributed without violating the octet rule, the greater will be the number of canonical forms.

Now we examine every option one by one.

Option A : $$SO_3$$
A common Lewis representation places one double bond between sulphur and each oxygen: $$O=S(=O)_2$$. That means three S-O double bonds, so there are three $$\pi$$-bonds, not one. Hence this option is rejected on the ground of the $$\pi$$-bond count.

Option B : $$O_2$$
Molecular oxygen is written as $$O=O$$, a single O=O double bond. Therefore it possesses exactly one $$\pi$$-bond. Concerning resonance, the ordinary Lewis picture has essentially a single dominant form; alternative forms involving charged or diradical species can be drawn but they are far less significant. Thus the practical number of important canonical forms is small (basically one), so this is not the species with the maximum number of canonical structures.

Option C : $$SO_2$$
The usual Lewis description shows two S=O double bonds, each contributing one $$\pi$$-bond, giving a total of two $$\pi$$-bonds. Therefore it fails the one-$$\pi$$-bond requirement and is ruled out.

Option D : $$CO_3^{2-}$$
Let us write one canonical form explicitly:

$$\displaystyle O^{-}\;-\;C(=O)\;-\;O^{-}$$

Here only the C=O linkage is a double bond, supplying one $$\pi$$-bond. Now, by moving the position of this double bond to the other two C-O connections, we can obtain two more equivalent Lewis structures. Thus:

1. $$O^{-}\;-\;C(=O)\;-\;O^{-}$$

2. $$O^{-}=C\;-\;O^{-}$$

3. $$O^{-}\;-\;C\;=\;O^{-}$$

So the carbonate ion has three canonical forms, all equally important and generated simply by rotating the location of the single $$\pi$$-bond. Among all the species that satisfy the “one $$\pi$$-bond” condition, $$CO_3^{2-}$$ possesses the largest number of significant resonance structures.

Therefore the species that fulfils both requirements—exactly one $$\pi$$-bond and the maximum possible number of canonical forms—is the carbonate ion, $$CO_3^{2-}$$, corresponding to Option D.

Hence, the correct answer is Option D.