Final answer:
Protozoan cysts are a dormant stage that forms in response to harsh environmental conditions and are highly resistant, but they are not reproductive structures. The correct answer to the student's question is E) both B and C.
Step-by-step explanation:
Protozoan cysts are known for serving specific functions within the life cycle of protozoa. These cysts are not reproductive structures; rather, they are a form of dormant stage that protozoa enter during unfavorable environmental conditions. Protozoan cysts are incredibly resistant to environmental conditions, which allows them to survive extreme temperatures, desiccation, and low pH levels. This resistance is a survival strategy, enabling the protozoans to wait until conditions improve or until they are carried to a more favorable environment.
Sexual reproduction in protozoans often correlates with the formation of these resistant cysts, allowing for genetic recombination as well as enhanced survival prospects in changing conditions. Consequently, the correct answer to the question is E) both B and C.