The correct answer is 3: 0.10 M K3PO4(aq). The higher the concentration (or molarity), the higher the boiling point. Thus, the solution with the most moles will have the highest boiling point. SO4, PO4, and NO3 are all polyatomic ions, so by definition, they only have one mole. In K3PO4, K3 has 3 moles and PO4 has 1 mole, meaning all together it has 4 moles (more than any of the other options).
