Which of the following aqueous solutions should have the highest boiling point ?

More the value of van't Hoff factor (i), more is the boiling point. 

For most of the ionic compounds dissolved in water, the van't Hoff factor is equal to the number of discrete ions in a formula unit of the substance.
$$\begin{gathered} \mathrm{NaOH}\to {\mathrm{Na}}^{+}+{\mathrm{OH}}^{-} ; \mathrm{i}=2 \\ {\mathrm{Na}}_2{\mathrm{SO}}_4\to 2{\mathrm{Na}}^{+}+ {\mathrm{SO}}_4^{-2}; \mathrm{i}=3 \\ {\mathrm{NH}}_4{\mathrm{NO}}_3\to {\mathrm{NH}}_4^{+}+{\mathrm{NO}}_3^{-} ; \mathrm{i}=2 \\ {\mathrm{KNO}}_3 \to {\mathrm{K}}^{+}+{\mathrm{NO}}_3^{-} ; \mathrm{i}=2 \end{gathered}$$