Which of the following structures in flatworms plays a role in excretion?

The flame cell is a specialized excretory structure found in flatworms, playing a key role in their excretory system. This structure is part of the protonephridial system, which helps maintain osmotic balance and removes nitrogenous waste from the organism's body. The flame cell has a cluster of cilia that create a flickering motion, resembling a flame, which helps to propel waste fluids into a series of tubules leading to excretory pores. This efficient filtration mechanism allows flatworms to eliminate soluble waste while conserving water, making it essential for their survival in various aquatic environments.

In contrast, the tegument is the outer covering of flatworms that serves primarily as a protective layer; it does not have a role in excretion. The scolex is a part of tapeworms, specifically designed for attachment to the host's intestinal wall, and it is not involved in excretory functions. The radula, found in some mollusks, is a feeding structure used for scraping food and is unrelated to excretion in flatworms. Thus, the flame cell is distinctly recognized as the correct answer for its vital role in the excretion process of flatworms.