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.