Animal Kingdom - Chordata

The phylum Chordata is defined in NCERT Class 11, Chapter 4 (Animal Kingdom) by three fundamental features present at least at some stage of the life cycle: a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits. These distinguish chordates from all non-chordates and remain diagnostic whether the animal is a fish, amphibian, reptile, bird or mammal. Hence the first option correctly lists them. The set naming a ventral solid nerve cord with an exoskeleton and jointed legs is wrong, because those are non-chordate features typical of arthropods; chordates have a dorsal hollow nerve cord, not a ventral solid one. The combination listing radial symmetry is incorrect, since chordates are bilaterally symmetrical, and a ventral heart alone is not part of the defining triad. The option mentioning a segmented shell is invalid, as a shell is not a chordate character at all. In addition to the three core features, chordates possess a post-anal tail, a closed circulatory system and a ventral muscular heart. A consistency check confirms the answer: only the first set contains all three universally accepted diagnostic chordate characters together, while every other option mixes in non-chordate traits that would never co-occur in this combination.