All flatworms differ from all roundworms in having
A. Triploblastic body
B. Solid mesoderm
C. Bilateral symmetry
D. Metamorphosis in life history

Hint: Platyhelminthes or the flatworms are the invertebrates whose body is unsegmented, soft, and bilateral. Platyhelminthes are acoelomates that mean it does not have a body cavity. They do not possess a specialized respiratory system or cardiovascular system and the exchange of oxygen takes place by diffusion through their body.

Complete answer: Platyhelminthes is commonly known as flatworms. Flatworms are bilaterally symmetrical animals. The body structure of a flatworm is dorsoventrally flattened. The outer body of flatworms is covered with cilia which is used for locomotion. Flatworms do not have body cavities. They possess a single opening which acts as both mouth and anus. Flatworms generally live in water bodies. Nematodes are commonly known as roundworms. Roundworms are cylindrical in shape and tapering at the ends. The body of roundworms is covered by a rigid covering called cuticles and they shed off as they grow. Roundworms consist of longitudinal muscles that are used to bend their bodies during locomotion. Roundworms are pseudocoelomate. It means they possess a body cavity between their mesoderm and endoderm layers. The body of roundworms shows the presence of a complete digestive tract which involves two openings: the mouth and the anus. Roundworms possess three embryonic layers of cells that are ectoderm, mesoderm, and endoderm which is absent in flatworms.
So, option B is the correct option.

Note: Some species of flatworms are hermaphroditic. It means they possess both male and female sex organs. The method of reproduction of flatworms is simple as compared to the roundworms. Roundworms have two distinct male and female roundworms. Both flatworms and roundworms are harmful to human beings.