Answer:
D) NaNO₃
Step-by-step explanation:
In this case, to know this, we need to watch the solubility curve of each compound to know which one is saturated or unsaturated in this case.
As you can see in the curve attached, let's analize first what is an unsaturated solution.
An unsaturated solution is a solution where the quantity of solvent is in excess and the solute is completely dissolved in solution leaving no remains. This depends of the temperature of course. The solubility of one compound vary with the increase or decrease of temperature.
As you can see in the graph attached, what happens to the KCl?, we can see that a 40 °C, you just need around g of solute to be dissolved, and be saturated so, this is not the correct choice.
If you watch NaCl, has a similar behavior. You just need about 35 g of solute to be saturated.
Finally the KNO₃ just need 60 g of solute to be dissolved in solution, completely.
In the case of NaNO₃ we can see that at 40 °C it's not touching the curve, so we can assume that this is an unsaturated solution and it would be the correct choice.