Answer: The correct answer is- A: allopatric speciation.
Allopatric speciation can be described as formation of new species due to geographical isolation of a population.
Darwin studied various species of finches on isolated galapagos islands.
They developed different adaptations (in the form of different beak shapes) due to different food availablilty.
Thus, in due course of time, different species evolved from common ancestor. This represented allopatric speciation.
Thus, option A) is the right answer.