Final answer:
The Grants' study of finches is significant because it offers a long-term observation of natural selection in action on the Galápagos Islands, particularly in changes in beak sizes of finches due to environmental pressures and food resource availability.
Step-by-step explanation:
The Grants' study of finches on the Galápagos Islands is an important contribution to the understanding of natural selection. Their long-term observation since 1976 has provided valuable insights into how environmental factors such as availability of food resources affect the evolution of species. Specifically, they observed a significant shift in beak sizes of the medium ground finches on Daphne Major after a drought. The large-billed birds could feed on large hard seeds, which were still available, while the small-billed birds could not, leading to a survival advantage for the large-billed birds. This resulted in an increased average beak size in the following generations, demonstrating evolution by natural selection.
Therefore, the correct answer to the student's question is D) It offers a long-term observation of natural selection in action. The evidence collected by the Grants shows how natural selection can drive changes in inherited traits such as beak size, based on environmental pressures and changes in food resources.