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

An increase in the boiling point of the solution depends upon the van’t Hoff factor (i) of the solute when it is added to the solution.

In Option 1-NaOH dissociates into 2 ions,

In Option 2-Na₂SO₄ dissociates into 3 ions,

In Option 3-NH₄NO₃ dissociates into 2 ions and

In Option 4-KNO₃ dissociates into 2 ions.

Van’t Hoff factor is the maximum for option (ii).

Thus, the maximum increase in the boiling point will be on the addition of 1M Na₂SO₄ in solution.

Thus, Option (ii) is the correct answer.