Ans.
Galapagos islands consist several small and large islands, which inhabit several organisms, including Galapagos finches. Darwin studied on the different species of those finches and explained theory of natural selection.
The primary differences between the Galapagos finches was difference in shape and size of their beaks, which were of great interest to Darwin.
This difference was developed due to natural selection as those finches became adapted to different food sources present in islands.
Thus, the correct answer is 'differences in size and shapes of beaks.'