Which of the following best defines a vertebrate?

The correct answer identifies vertebrates as animals characterized by having an internal skeleton and a segmented spine. Vertebrates belong to the phylum Chordata, and one of their defining features is the presence of a backbone or vertebral column, which provides structural support and protects the spinal cord. This internal skeleton is composed of bone or cartilage, allowing for greater mobility and support compared to animals with an external skeleton.

In contrast to the other options, having an external skeleton pertains to invertebrates such as arthropods, and being warm-blooded or cold-blooded relates to thermoregulation rather than skeletal structure. These aspects do not define whether an animal is a vertebrate or not, as they can apply to various animal groups outside of the vertebrate classification. Therefore, the emphasis on the internal skeleton and segmented spine is essential in accurately defining vertebrates within the animal kingdom.