Which characteristic is common among vertebrates?

The presence of a segmented spinal column is a defining characteristic of vertebrates. This spinal column, or vertebral column, is made up of individual vertebrae that provide support for the body, protect the spinal cord, and facilitate movement. The segmented nature of the spinal column allows for flexibility and a variety of movements, which are essential for the diverse lifestyles that vertebrates lead, whether in water, on land, or in the air.

The other characteristics presented in the options do not apply universally to all vertebrates. While some vertebrates have adaptations for breathing underwater, such as gills in fish, not all vertebrates possess this ability, particularly terrestrial ones. Similarly, vertebrates do not exhibit a lack of a spinal column, which would disqualify them from being classified as such. Lastly, an external skeleton, or exoskeleton, is present in invertebrate groups like arthropods, but vertebrates have an internal skeleton structure. Thus, the characteristic of having a segmented spinal column is uniquely and consistently associated with all vertebrates.