Final answer:
The physical feature that distinguishes earthworms from flatworms and roundworms is Segmentation. Earthworms have a segmented body plan that allows for increased flexibility and more developed organ systems, unlike the acoelomate flatworms and pseudocoelomate roundworms.
Step-by-step explanation:
The physical feature that makes earthworms different than flatworms or roundworms is A. Segmentation. Segmentation refers to a division of the body into multiple repeats called segments. Earthworms, belonging to the phylum Annelida, exhibit a segmented body plan known as metamerism. This differs significantly from flatworms, which are acoelomate (having no true body cavity) and exhibit an incomplete digestive system, and roundworms, which have a pseudocoelom yet lack segmentation. Annelids like earthworms possess a true coelom and organ systems that are more developed, including a complete digestive system with two openings and five pairs of hearts that pump blood along the two main vessels. Their bodies have multiple segments that increase flexibility and allow for a wider range of motion, an advantage not present in the unsegmented flatworms and roundworms.