Final answer:
Roundworms have a pseudocoelom, which is a body cavity partly lined by mesoderm and endoderm, making them pseudocoelomates (option B).
Step-by-step explanation:
Roundworms are classified as pseudocoelomates, which means their body cavity is not entirely lined with mesoderm. Instead, it is partly lined by mesoderm and partly by endoderm. This type of body cavity, known as a pseudocoelom, is partially separated from the digestive cavity and allows room for the internal organs to develop. It also supports the body by forming a hydrostatic skeleton, contributing to the characteristic round shape of roundworms. However, this is different from true coelomates, or eucoelomates, in which the coelom is completely surrounded by mesodermal tissue. Therefore, the correct answer to the student's question is B. Pseudocoelomate.