This answer represents the weakest acid in this group. All of these acids have the acidic hydrogen atom on an oxygen atom. Therefore periodic table effects are not involved in the decision as to which is the strongest acid. The atom to which the hydroxyl group is attached is different in each case. The first step that you should do in order to answer this question would be to draw out the electron dot formulas for each compound and then calculate the formal charge on the atom to which the hydroxyl group is bonded. When this atom has a significant amount of positive charge, it will stabilize the anion formed when the hydroxyl group loses a proton. In the case of acetic acid, the carbon atom only has a partial positive charge due to the dipole of the C=O group based on electronegativity. All of the other acids have full positive formal charges and thus will be stronger acids.