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• Chordates includes vertebrates and two groups of invertebrate chordates: Cephalochordata and Urochordata.
• Animals in the phylum Chordata share five key characteristics that appear at some stage during their development: a notochord, a dorsal hollow (tubular) nerve cord, pharyngeal gill arches or slits, a post-anal tail, and an endostyle/thyroid gland.

Cephalochordates possess a notochord, dorsal hollow tubular nerve cord, pharyngeal slits, endostyle/thyroid gland, and a post-anal tail in the adult stage. As they show all the five key features of chordates. This makes Cephalochordates the most closely related to chordates. Hence, option (a) is the right answer.

b. echinoderms

Echinoderms include invertebrate marine animals that have pentaradial symmetry and possess a spiny body covering. They do not possess any of the five characteristic features of chordates. Hence, option (b) is the incorrect answer.

c. arthropods

Arthropods are invertebrates that are characterized by an exoskeleton, a segmented body and paired appendages. They do not possess any of the five characteristic features of chordates. Hence, option (c) is the incorrect answer.

d. urochordates

Even though urochordates are considered chordates, the notochord, dorsal hollow nerve cord and post-anal tail is absent during the adult stage. Only pharyngeal slits and endostyle are present during the adult stage. Due to these reasons, urochordates are not as closely related to chordates compared to cephalochordates. Hence, option (d) is the incorrect answer.