Sac-like structures under the plasma membrane are a characteristic of which major group of protists?

The answer is alveolates. The sac-like structures under their plasma membrane are called alveoli. The alveoli provide support to the plasma membrane. Examples of groups of alveolates are ciliates, such as Paramecium, dinoflagellates, apicomplexa, and foraminifera. They lie under the phylum, Ciliophora.