The strength of an acid is determined by its tendency to donate a proton (H+). The more stable the conjugate base of an acid, the stronger the acid. In this case, we can compare the stability of the conjugate bases of the given acids by considering the resonance and inductive effects of the iodine atom.
Option A: 3-iodobutyric acid - The negative charge of the conjugate base will be delocalized through resonance, making it more stable than the conjugate bases of options B, C, and D.
Option B: 4-iodobutyric acid - The negative charge of the conjugate base will be delocalized through resonance, but less effectively than in option A due to the distance of the iodine atom from the carboxylic acid group.
Option C: 2-iodobutyric acid - The negative charge of the conjugate base will be localized near the iodine atom, making it less stable than the conjugate bases of options A and B.
Option D: butyric acid - This molecule does not contain an iodine atom, so its conjugate base will be less stable than the conjugate bases of options A and B due to the absence of the electron-withdrawing effect of iodine.
Therefore, option A (3-iodobutyric acid) is the strongest acid of the given compounds.