Which of the following is a distinctive characteristic of Alveolates?

Alveolates are defined by the presence of membrane-enclosed sacs called alveoli, located just beneath their plasma membrane. This unique structural characteristic plays a critical role in their physiology and functions, contributing to various aspects of their biology, including buoyancy and stability. The alveoli are thought to help maintain the cell's shape and may also play a role in osmoregulation or other cellular activities.

Other options describe features that pertain to different groups of organisms or do not accurately represent the Alveolates. For instance, the presence of rigid cell walls is characteristic of certain groups like plants and fungi, but Alveolates have a flexible cell membrane instead. Radial symmetry is more commonly associated with organisms like cnidarians rather than with Alveolates, which can exhibit a variety of body plans. Additionally, while binary fission is a mode of reproduction for many protists, it is not exclusive to them; other reproductive strategies exist within the group as well. Therefore, the defining feature of Alveolates is indeed the presence of those unique membrane-enclosed sacs, or alveoli.