Final answer:
The Branchiopoda is the group of Crustacea that has the most ancestral features, including a long trunk with uniform appendages, indicative of more primitive characteristics within the subphylum.
Step-by-step explanation:
The group of Crustacea with the most ancestral features, including a long trunk with fairly uniform appendages, is the Branchiopoda. Branchiopods typically have characteristics that are considered more primitive or similar to the early ancestors of the Crustacea.
This includes biramous appendages, which are believed to represent the ancestral condition in arthropods, similar to how it was observed in the now-extinct trilobites. Contrasting to other crustacean groups like Malacostraca, which includes animals like crabs and lobsters, and Copepoda, which are an important group of zooplankton, the Branchiopoda retain more of these primitive features.