Final answer:
Echinoderms are considered to be the most closely related to basal chordates because they share a common ancestor with chordates, belonging to the superphylum Deuterostomia and exhibiting similar embryonic development patterns.
Step-by-step explanation:
The extant group likely to be the most closely related to the basal chordates are the echinoderms because they share a common ancestor with chordates. This is evidenced through both groups belonging to the superphylum Deuterostomia and sharing similar patterns of early embryonic development. The characteristic features such as a notochord, a dorsal nerve cord, and pharyngeal slits are present in chordates and to some extent in hemichordates, a close relative of echinoderms. Although echinoderms do not have notochords, adults exhibit pentaradial symmetry, and have an endoskeleton made up of ossicles, it is the development pattern which places them closer to chordates than other invertebrate phyla like protostomes, such as mollusks, annelids, and arthropods.
In contrast, features such as radial symmetry and a water vascular system are unique to echinoderms and do not support a close relationship with chordates. The key takeaway here is that the close relationship is based on embryological development and genetic lineage rather than the presence of specific physical structures like a notochord or water vascular system in adulthood.