Which salt solution could contain 42 grams of solute per 100 grams of water at 40°c?

unsaturated solution of NH4Cl

Explanation:
To answer this question, you will need to refer to the solubility curves reference diagram (I have attached this diagram).

The curve shows the temperature on the x-axis and the solute per 100 grams of water on the y-axis.

So, you would simply check the diagram and find the intersection point between a temperature of 40 degrees Celsius and a 42 gram solute per 100 grams of water (the value is expected to be somewhere between 40 and 50 of course)

Doing this, you would find that the closes t solution fitting the required criteria is the unsaturated solution of NH4Cl

Hope this helps :)