Final answer:
Echinoderms (Option A) are the most closely related animal group to chordates, sharing a common deuterostome ancestry and similar embryological development patterns.
Step-by-step explanation:
The animal group most closely related to chordates is echinoderms (Option A).
The relationship between echinoderms and chordates is based on both groups being part of the superphylum Deuterostomia. In deuterostomes, the mouth develops secondarily to the blastopore, which becomes the anus. This developmental sequence differs from that of protostomes, where the mouth develops from the blastopore. This distinction between protostomes and deuterostomes is a major factor in determining evolutionary relationships among animal phyla.
Furthermore, both echinoderms and chordates exhibit a complex level of organization, sophisticated tissue structures, and a well-developed coelom. Molecular studies also support this relationship, as evidence indicates that echinoderms and chordates share a more recent common ancestor compared to other invertebrate groups.
Echinoderms, such as sea stars and sea urchins, while being invertebrates and physically quite distinct from chordates, share similar patterns of embryonic development with chordates. Such embryological evidence further establishes the closer kinship between these two phyla over others like mollusks, annelids, and arthropods, all of which are protostomes. Therefore, in the context of evolutionary biology, echinoderms are the animal group most closely related to chordates.