Which group includes fish, amphibians, reptiles, birds, and mammals?

• A. Vertebrates
• B. Mammals
• C. Agnatha
• D. Chordates

Answer: (D)

The main idea tested is recognizing a group defined by having a backbone. Fish, amphibians, reptiles, birds, and mammals all share a vertebral column that protects the spinal cord and supports the body, so they are all vertebrates. Vertebrates are a subset of chordates, which also include some animals that don’t have backbones, like certain tunicates and lancelets. That broader chordate group would include more than just these five classes, so vertebrates is the best fit for describing the group that unites all of them specifically by the presence of a backbone. The other options are either too narrow (mammals) or too broad (chordates) to correctly capture the set of five classes listed.