The organisms examined by darwin on the galápagos islands that were most important in his development of the theory of natural selection were:

The correct answer is finches

Explanation:

Darwin when went to Galapagos islands he observed that the finches that live on these islands closely resemble the the mainland finch species of South America. He saw the differences in the beak structures of these island finches.

After observing these finches he hypothesized that the species of finches might be evolved from the mainland finches after inhabited the islands. He analyzed that the different beak structure and size of finches help them to eat different foods and reduce competition.

So by studying finches, he developed the theory of natural selection which says that those organisms will survive and reproduce which are better adapted to their environment.

the answer to your question is slate