Final answer:
The correct answer is A) a whale's pectoral fin and a bird's wing, as both structures are homologous, indicating a common evolutionary past.
Step-by-step explanation:
The question asks which set of structures are homologous. Homologous structures are those that share a similar embryonic origin and indicate a common evolutionary past, even though they may have different functions. Therefore, in terms of homology:
- A whale's pectoral fin and a human arm are homologous because they share similar bone structures despite having different functions.
- A bird's wing and a bat's wing are homologous as well as analogous because they share common structural features that arose from a common ancestor and serve the same function of flight.
- Butterfly wings and bat wings are analogous due to similar functions (flight) but are not homologous since they do not share a common embryonic origin.
- A moth's eyes and a cow's eyes may have a similar structure and function, but this choice doesn't illustrate the principle of homology as clearly as some other examples.
- A beetle's leg and a horse's leg are not homologous structures. They don't share the same evolutionary origin.
Thus, from the provided choices, the set of structures that are homologous is A) a whale's pectoral fin and a bird's wing.