Final answer:
A jellyfish is most likely to be a diploblast, characterized by having two embryonic germ layers and typically showing radial symmetry, unlike flatworms, roundworms, and earthworms which have three layers and bilateral symmetry.
Step-by-step explanation:
The organism most likely to be a diploblast is a jellyfish. Diploblastic organisms develop two embryonic germ layers: the ectoderm and the endoderm (or mesendoderm). Jellyfish, which belong to the phylum Cnidaria, typically show radial symmetry and are diploblastic. In contrast, flatworms, roundworms, and earthworms are all triploblastic with three germ layers (ectoderm, mesoderm, and endoderm) and display bilateral symmetry. Moreover, flatworms are acoelomate, lacking a body cavity, whereas roundworms are pseudocoelomates with a body cavity that is not completely lined with mesoderm. These traits indicate a higher level of complexity compared to the simplistic diploblastic organization of jellyfish.