Answer:
D. Ecological competition
Step-by-step explanation:
The small population of finches reached the Galapagos island and established themselves. The geographical isolation of Galapagos islands from each other led to the prevention of interbreeding among the populations of finches present on the separate islands.
The different populations of finches accumulated variations to get adapted to the available food. Accumulation of genetic variations and geographical isolation led to their reproductive isolation from each other.
To get themselves better adapted to the surroundings, the population of finches experienced increased genetic differences between them, that is ecological competition. Repetition of the whole process of the chains of islands led to the evolution of different species of finches.