Answer: B. Solution
Explanation: According to the question statement, Susan wrote in her notebook that it was a blue liquid. Therefore, from a chemistry point of view, it was a solution and not a heterogeneous mixture. Then, the solvent was evaporated after leaving the solution for 72 hours at the windowsill. The solute (blue color most likely) however was not able to evaporate with solvent and left behind.
Example: if you leave the KMnO4+water solution for a few days, water will evaporate thus leaving behind purple crystals of KMnO4.
