Concept:Vestigial organs are remnants of structures that were fully functional in ancestral forms but have lost their original function in some species.Explanation:In humans, the appendix is considered vestigial because it has no major digestive role.However, in herbivorous mammals, the appendix (or caecum) is functional and helps digest cellulose.The liver, rectum, and stomach remain fully functional in humans and other mammals.Therefore, the appendix best matches the condition described.Answer:B. Appendix