Final answer:
Contractile vacuoles act as excretory systems in microorganisms like amoebas by actively pumping excess water and waste out of the cell through a process called exocytosis. This helps maintain osmotic balance within the freshwater environment where they typically reside.
Step-by-step explanation:
Contractile vacuoles in microorganisms such as amoebas serve as excretory systems. The answer to the student's question is a) By actively pumping water out of the cell. These specialized vacuoles help to manage the osmotic balance within the cell by collecting and expelling excess water and waste. This process is known as exocytosis, where the vacuole membrane fuses with the cell membrane, thereby releasing waste into the surrounding environment. Contractile vacuoles are particularly important in freshwater environments, where the osmotic pressure can cause an influx of water into the cell.
Other excretory structures in different organisms include flame cells and nephridia in worms, as well as Malpighian tubules in insects. These systems also function to remove waste and maintain osmotic balance but employ different mechanisms appropriate for their specific organism and environment.