1. A dorsal, hollow nerve cord.
2. A notochord at some time during development.
3. Pharyngeal gill slits at some time during development.
4. A muscular post-anal tail at some during development.
- The Diaphragm is a dome-shaped muscular partition that separates the thorax from the abdomen. Its presence is a characteristic feature of only mammals.
So, the correct option is 'Diaphragm'.