Question 2
In activity 3.1.8, sugar and salt are added to equal volumes of water at the same temperature and unequal masses dissolve. What does that establish?
The right answer is the one marked above.
The nature of the solute affects solubility. The solvent, the volume and the temperature were deliberately kept the same in both beakers, so the solute is the only thing left that could account for the difference.
The design of the activity is what makes the conclusion safe. Change two things at once and the result tells you nothing about either.