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 nitrate anion has a formal charge of +1 on the nitrogen atom as well as 2 oxygen atoms which all pull electron density from the nitrogen due to their high electronegativity and this exerts a strong pull on the electron pairs of the oxygen atom with the negative charge. The nitrate ion also has good resonance delocalization of the negative charge which makes the electron pairs less available for sharing.