This answer represents the weakest base in this group. Remember that the conjugate base of a strong acid must be a weak base or the acid would not be able to give up the proton. You can answer this question by thinking about...what are the acids that form these bases and which of these acids is the weakest? That conjugate base of that acid will be the strongest base...it competes best for the proton. Sulfuric acid is the strongest acid in this group of compounds and thus sodium hydrogen sulfate will be the weakest base.
All of these anions are stabilized by the positive atom next to it...the positive atom attracts the electrons of the oxygen atom and makes them less available for the reverse reaction. The first step that you should do in order to answer this question would be to draw out the electron dot formulas for each anion and then calculate the formal charge on the atom to which the oxygen atom with the negative charge is bonded. When this atom has a significant amount of positive charge, it will stabilize the anion. This is only part of the reason but it suffices for this course.