Final answer:
The genus of protists that contradicts the idea that unicellularity restricts cell size is Ulva, due to its large, multicellular structure, which deviates from the common small scale of most protists.
Step-by-step explanation:
The genus of protists that appears to contradict the statement that unicellularity restricts cell size is Ulva. This particular protist, also known as sea lettuce, is a large multicellular green algae. Generally, protist cells are tiny and can range in size; however, Ulva challenges this generalization with its large, edible, green blades. While most protists such as Acanthocystis turfacea and Tetrahymena thermophila are microscopic, Ulva represents a larger form within the Chlorophyta, diversifying from unicellular to various levels of multicellular forms.
Moreover, another protist that boasts large cell size is Caulerpa, a seaweed which consists of multinucleate cells that can reach up to 3 meters in length. Hence, these examples show that not all cells are confined to small sizes and that there are exceptions to common assumptions about cell size limitations in unicellular organisms.