Which one of the following aqueous solutions will exhibtt highest boiling point?

The elevation in boiling point is a colligative property of the solution. i.e. elevation in boiling point is directly proportional to solute particle concentration.

Consider, the dissociation of Sodium sulphate:

${\mathrm{Na}}_2{\mathrm{SO}}_4\to 2{\mathrm{Na}}^{+}+{\mathrm{SO}}_4^{2-}$

Therefore, concentration of ions in $0.01 \mathrm{M} {\mathrm{Na}}_2{\mathrm{SO}}_4$ solution is $3\times 0.01=0.03 \mathrm{M}$ In option A and D, Urea and Glucose both are covalent molecules and does not undergo dissociation therefore the concentration of solute particles in both solution is 0.015 M.

Consider, the dissociation of ${\mathrm{KNO}}_3$

${\mathrm{KNO}}_3\to {\mathrm{K}}^{+}+{\mathrm{NO}}_3^{-}$

Therefore, concentration of ions in $0.1 \mathrm{M} {\mathrm{KNO}}_3$ solution is $2\times 0.01=0.02 \mathrm{M}$ Hence, the highest number of solute particles in  $0.01 \mathrm{M} {\mathrm{Na}}_2{\mathrm{SO}}_4$ solution.  Therefore, $0.01 \mathrm{M} {\mathrm{Na}}_2{\mathrm{SO}}_4$ solution will show the highest boiling point.

Final Answer: Option A.