The correct answer would be: B. Coelomates have a fluid-filled body cavity with a peritoneum, derived from mesoderm, that separates the organs from the body fluid on one side and a muscle layer around the organs on the other side.
When going through the other options, we can recognize that option A is incorrect because aceolomates have no body cavities! After all, the prefix "a-" indicates a lack!
Option C is incorrect because, even though pseudocoelomates DO have a fluid-filled body cavity, it is NOT fully lined by the mesoderm. That is why they are called PSEUDOcoelomates.
Option D is also incorrect because pseudocoelomates' body cavities are fluid-filled, not filled with mesenchymal tissue as acoelomates are.
Try to remember:
Acoelomates: No body cavity, organs in mesenchyme.
Pseudocoelomates: Fluid-filled cavity, partially lined by mesoderm.
Ceolomates: True body cavity, fully lined by mesoderm/peritoneum.
