Which of the following is an example of vestigial structure?

A vestigial structure is an anatomical feature that has lost most or all of its original function through the course of evolution. These structures were functional in ancestral species but have become reduced or purposeless in the descendant organism.

The human appendix is a classic example of a vestigial structure. In herbivorous ancestors, the caecum (to which the appendix is attached) was large and played an important role in digesting cellulose from plant material. In modern humans, the appendix is a small, narrow tube attached to the caecum that no longer serves a significant digestive function. While recent research suggests it may have a minor role in immune function and gut flora, it is dramatically reduced compared to its ancestral form and can be surgically removed without noticeable consequences.

The other structures listed are all fully functional in their respective organisms. The elephant trunk is a highly developed, multi-purpose organ used for breathing, drinking, grasping, and communication. Bird wings are essential for flight (or, in flightless birds, have been repurposed for balance or swimming). Fish gills are the primary respiratory organs, extracting dissolved oxygen from water. None of these has lost its original function.

Exam tip: Vestigial structures provide evidence for evolution because they indicate common ancestry with organisms in which the same structure is fully functional.