Final answer:
The statement that rudist clams were the dominant reef builders during the Early Cretaceous is true. Rudists were a form of bivalve mollusk that grew either solitarily or in reefs during this period until their extinction at the end of the Cretaceous. This statement is A. True.
Step-by-step explanation:
During the Early Cretaceous, rudist clams were indeed the dominant reef builders. Rudists were a type of bivalve mollusk that existed from the Late Jurassic to the Cretaceous period. These unique and highly diversified organisms had two very unequal shells and could grow either solitarily or in vast colonies that formed reefs. Although the Cambrian period is known for the abundance of trilobites and reef-building sponges, it was during the Cretaceous when rudists took center stage in reef construction. The appearance of scleractinian corals, or true stony corals, in the Middle Triassic marked the beginning of their long-standing role in the history of reef formation, but it was the rudists in the Cretaceous that were particularly influential as reef builders before they went extinct at the end of that era.