Answer:
a. the beaks of finches
Explanation:
Charles Robert Darwin visited Galápagos Islands and observed many species of finches which had variation in the type of beaks they had. Finches residing in different regions of the islands had different kind of beaks. Then he came to the conclusion that the variation in the type of beaks is based on the type of niches they are residing because in different regions different kind of food is available. Although initially all the finches had same type of beaks yet later when some of them migrated to other regions of the island, they adapted their beaks in accordance with the type of food which was available in the new habitat. This adaptation in the beak shapes and sizes made them fit to survive in the new habitat. This observation helped Darwin to develop his theory of evolution which states that the organism which is more fit according to the environment will survive and will be able to continue generations while the organism which is less suited to the environment will die out.
