Which term describes organisms with a notochord at some life stage, including humans?

• A. Cetacea
• B. Cnidaria
• C. Chondrichthyes
• D. Chordata

Answer: (D)

The main idea is that having a notochord at some point in development is the defining feature of the phylum Chordata. In chordates, an early embryo has a flexible rod along the back called the notochord, which provides support and helps guide the development of the nervous system. In many vertebrates, including humans, this structure is present during development and is later replaced by the vertebral column, but its presence at any life stage still places the animal in Chordata. The other groups listed don’t capture this broad, defining trait: one lacks the notochord entirely, and the other options are specific subgroups within chordates rather than the broad category that includes all organisms that have or had a notochord.