Which definition best describes a vertebrate?

The definition that best describes a vertebrate is the one that highlights the key characteristic of these animals: the possession of a backbone or spinal column. Vertebrates, which include groups such as mammals, birds, reptiles, amphibians, and fish, are specifically classified based on this anatomical feature. The backbone is crucial for protecting the spinal cord and supporting the structure of the body, which distinguishes vertebrates from invertebrates, which lack this feature.

The other options do not accurately capture the essence of what defines a vertebrate. For example, the first option refers to animals without a backbone, which actually describes invertebrates, not vertebrates. The second option limits the definition to warm-blooded animals classified as mammals, which excludes other vertebrates like reptiles and fish. The last option states that none of the choices are correct, but since there is a correct definition provided, this option also fails. Therefore, the correct answer encompasses the fundamental characteristic that is unique to vertebrates, making it the best choice.