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?

Competition for available food resources (plants, seeds, insects, cactus etc)  among themselves.

Explanation:

The ancestral finches were the seed eating stock that fed on seeds. These ancestral finches diversified according to different types of food due to competition for available food resources (plants, seeds, insects, cactus etc) among themselves. This led to diversification of ancestral stock and evolution of unique beak size and shape in the finch population of each of the Galapagos Islands.

well, i believe that the answer is 2. : 3

what are the options? add them, then i will be glad to .