S=O double bonds

Similar arguments can be used for these molecules as in part (a). In SO2 and SO3, the Lewis structures with the lowest formal charge at S have double bonds between S and each O. In the sulfite ion, however, there are three Lewis structures that have a zero formal charge at S. Each has one S—O double bond and two S—O single bonds. Because these S—O bonds would have a substantial amount of single bond character, they would be expected to be longer than those in SO2 or SO3. This is consistent with the experimental data that show the S—O bond lengths in... 

The correct answer is (D). SO, has three resonance structures. The concept of resonance explains why all three of these bonds are identical as opposed to having one shorter S=O double bond and two longer S-O single bonds. 

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