This answer is actually the weakest acid of the compounds shown. I think that you are confused as to what constitutes a good acid. First, the acidic hydrogen atom must have a reasonable amount of positive character (proton character) and this is caused by having this atom attached to a more electronegative atom. Remember that the greater the difference in electronegativity between two atoms, the greater the dipole. Secondly, ideally there should always be some stabilization of the anion that is formed after a base reacts with the acidic hydrogen atom. Think about these concepts and select another answer.