Answer:
C. Beak shape in finches varies according to the food source available on each island.
Step-by-step explanation:
During his visit to the Galápagos Islands, Darwin noticed that the finches on different islands had distinct beak shapes that seemed to be adapted to the specific types of food available on each island. This observation led him to propose that the beak shape of finches had been modified over time through natural selection in response to the specific food sources present on each island. This idea became a crucial component of Darwin's theory of evolution by natural selection, demonstrating how variations in traits within a population can lead to differential survival and reproductive success based on the environment and available resources.