Charles Darwin observed a unique beak size and shape in the finch population of each of the Galapagos Islands that he visited. Which is the most likely cause of the observed variation?

In this question, there are no options given to choose from. So i would answer this question based on my knowledge. The most likely cause of the observed variation is adaptation and evolution. The birds of that island adapted to the food source and then they evolved as per the requirement of the place and food source.