The correct answer is - B) founder effect.
The founder effect represents the loss of genetic variation in a new population due to the small number of individuals from the larger population that have formed the new population.
The finches on the Galapagos Islands have developed unique characteristics in accordance to their environment. Because there were only very small number of individuals that settled each island, the genetic variation was not the same as while they were part of the larger populations.
Because of this, those small number of individuals starter to develop unique traits only on that single island, which can easily be seen in the shape of their beaks in accordance to the available food source.