Colligative property is an extensive property which depend on the number of moles of solute particles.
Aqueous Solution
Number of Particles
NaOH→Na++OH−
2
Na2SO4→2Na++SO42−
3
NH4NO3→NH4++NO3−
2
KNO3→K++NO3−
2
Since, the concentration of the all compounds are same, therefore, compound having maximum no. of ions, that is, Na2SO4 will have highest boiling point.