Final answer:
The answer is C: Gill slits in fish develop into gills used for breathing underwater, while in birds, these structures are present in the embryo but are repurposed and lost as the bird develops. This embryonic similarity indicates a common ancestry among vertebrates.
Step-by-step explanation:
The correct answer to the student's question is C: Gill slits develop into gills in fish, but as birds develop they lose their gill slits. During the embryonic development of chordates, which includes both fish and birds, structures known as pharyngeal pouches appear, which are commonly but inaccurately referred to as gill slits. In fish, these structures do indeed develop into gills, which are used for breathing underwater. However, in birds, these pharyngeal pouches do not develop into gills; instead, the structures are repurposed during development. For example, in later stages, the tissue between the gill slits in mammals forms part of the lower jaw and the bones of the inner ear; birds have similar developmental repurposing.
Birds are part of the terrestrial vertebrate group, and as they progress through development, they lose these early gill slits, just as humans and other terrestrial animals do. Conversely, fish retain these structures, which develop into fully functional gills that are used for oxygen uptake in water. The presence of these structures during the early stages of development provides evidence of a common ancestry among vertebrates, as seen through the lens of comparative embryology.