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 dihydrogen phosphate anion has a formal charge of +1 on the phosphorus atom as well as 3 oxygen atoms which all pull electron density from the phosphorus due to their high electronegativity and this exerts the second strongest pull on the electron pairs of the compounds shown. Another way to remember this is that the dihydrogen phosphate ion will be the second weakest base because phosphoric acid is the second 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.