Final answer:
Circle (b) best represents the unit volume after 100 mL of solvent has been added.
Step-by-step explanation:
Based on the information provided, circle (b) represents the unit volume after 100 mL of solvent has been added. The description states that 100 mL of water gives 100 mL of solution, meaning that when 100 mL of solvent is added to the solution, it will have the same volume. Therefore, circle (b) is the best representation of the unit volume after adding the solvent.