Biology

The correct answer is: A.His observations of finch species in the Galápagos Islands.

Explanation:

Darwin's main evidence for proposing gradualism came from his observations of different species of finches on the Galápagos Islands. He noted that these finches, although similar in appearance, had different beak shapes and sizes depending on the island and the food sources available. These small variations suggested that species evolved gradually over time through adaptation to different environments, supporting his theory of gradualism.