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.
