Missing question:
(1) the solution is diluted with water.
(2) some of the NaNO3(aq) solution is removed.
(3) the solution is stirred.
(4) more NaNO3(s) is dissolved in the solution.
Answer is: (4) more NaNO3(s) is dissolved in the solution.
Unsaturated solution means that more of a substance can be dissolve (in this case sodium nitrate). If more sodium nitrate is dissolved, molality of solution is increased, eo ipso boiling point of solution.
