Which of the following aqueous solution has the highest boiling point?

Boiling point elevation depends on the solvent, but we're assuming water, and upon the concentration of the solute and how many particles into which the solute breaks. Colligative properties like boiling point elevation, depend on the total number of particles in the solution. The solvent used here is water. So elevation in boiling point will be above the boiling point of water for all solutions.

$\mathrm{\Delta T}=\mathrm{i} \times {\mathrm{K}}_{\mathrm{b}} \times \mathrm{m}$, where i = van Hoff's factor, 
$K_b$= boiling point elevation constant, 
m = molality

Solutions with higher ΔT will have a higher boiling point.

For $0.1 \mathrm{M} {\mathrm{KNO}}_3$ (i) = (1+(n−1)α)=2  (α for salt is 1)

For $0.1 \mathrm{M} {\mathrm{Na}}_3{\mathrm{PO}}_4$ (i) = 4   (α for salt is 1)

For $0.1 \mathrm{M} {\mathrm{BaCl}}_2$  (i) = 3  (α for salt is 1)

For $0.1 \mathrm{M} {\mathrm{K}}_2{\mathrm{SO}}_4$  (i) = 3  (α for salt is 1)

Thus, Option B $0.1 \mathrm{M} {\mathrm{Na}}_3{\mathrm{PO}}_4$  has the highest Boiling Point