Final answer:
Parasitic flatworms would be grouped together based on the tegument as a synapomorphic feature.
Step-by-step explanation:
The organisms that would be grouped together based on the tegument as a synapomorphic feature are parasitic flatworms. The tegument is a specialized outer covering found in flatworms, which helps them in nutrient absorption, protection, and evasion of the host immune system. This feature is unique to flatworms and is not present in mammals, fish, or reptiles.