Final answer:
Poriferans, or sponges, are considered the most closely related to the common animal ancestor, due to their simpler organization lacking true tissues and organs, similar to what might have been the case for early animal life forms.
Step-by-step explanation:
Poriferans (sponges) are thought to be the most closely related to the common animal ancestor. This is due to their simple level of organization, as they lack true tissues and organs, and resemble a colonial form that might have been present in early stages of animal evolution. In contrast, other groups such as Cnidarians, Platyhelminthes (flatworms), and Annelids (segmented worms) show more complex levels of organization, with true tissues and organ systems. Cnidarians, like jellyfish, have true tissues but still only have two tissue layers and radial symmetry, and are more complex than Poriferans but not as complex as Bilateria, which includes both Platyhelminthes and Annelids.
Molecular evidence supports the contention that arthropods are more closely related to nematodes, forming the ecdysozoan clade, whereas annelids are part of the lophotrochozoan clade along with mollusks and other phyla. This means Cnidarians and Poriferans are not part of the Bilateria clade and are thus less related to the common bilateral ancestor of Platyhelminthes and Annelids.