Which of the following best describes chordates?

The description of chordates as organisms that possess a notochord at some stage of development is accurate. The notochord is a flexible, rod-shaped structure that provides support and is a defining feature of the phylum Chordata. It is present in all chordates during some part of their life cycle, although in some groups, like vertebrates, it may be replaced by a backbone in adults.

This characteristic is crucial because it sets chordates apart from other animal phyla. In addition to the notochord, chordates typically have a dorsal nerve cord, pharyngeal slits, and a post-anal tail at some stage in their development, which help further define the group. The presence of the notochord is integral to their development and evolution, making it the most appropriate choice to describe chordates.

The other options suggest incorrect or overly restrictive characteristics that do not apply to all members of the phylum, whereas the notochord is universally recognized as a fundamental aspect of chordate biology.