When a living plant is moved from a freshwater aquarium to a saltwater aquarium, the environment surrounding the plant changes from a low concentration of solutes (freshwater) to a high concentration of solutes (saltwater). This situation creates a hypertonic environment for the plant.
In a hypertonic environment, water moves out of the plant's cells to try to balance the concentration of solutes inside and outside the cells. As a result, the plant's cells would lose water.
Therefore, the correct answer is:
d The plant's cells would lose water.