Final answer:
Chordates are characterized by the unique feature of a dorsal, hollow nerve cord, along with a notochord, pharyngeal slits, and an endostyle/thyroid gland. This nerve cord sets them apart from non-chordate animals and is essential for the central nervous system. The correct option is 2.
Step-by-step explanation:
One characteristic that separates chordates from all other animals is the presence of a dorsal, hollow nerve cord, which is part of the central nervous system. The phylum Chordata is defined by this feature along with a notochord, pharyngeal slits, and an endostyle/thyroid gland that secretes iodinated hormones.
These structures are present at some point during the development of all chordates, which include both invertebrates like tunicates and lancelets, and vertebrates. Among the choices provided, a true coelom, blastopore becoming the anus, bilateral symmetry, segmentation, and a dorsal hollow nerve cord, the one characteristic unique to chordates is the dorsal, hollow nerve cord. Hence, 2 is the correct option.