This answer is not correct. All of these bases have either a full positive charge or a positive dipole on the atom adjacent to the oxygen atom that has the negative charge. This will attract the electrons on the charged oxygen atom and make them less available for sharing. The hydrogen sulfate anion has a formal charge of +2 on the sulfur atom and exerts the strongest pull on the electron pairs. Another way to remember this is that the hydrogen sulfate ion will be the weakest base because sulfuric acid is the strongest acid in the group. A strong acid must logically create a weak conjugate base or the acid could not give up the proton readily.