The diagram doesn't show any external interferences, such as mutations (the populations all have the same types of individuals) or recombination (there's no new types of indivudals or combination of those types). Therefore, D and A are incorrect.
There is a separation of the first population, so only natural selection wouldn't explain the outcome, therefore C is incorrect as well.
The genetic drift occurs when there are random fluctuations in a population. In smaller populations, such as this one, this random fluctuations can lead to an event called Founder event. In this case, an isolated founder population creates descendants that are selected and different due to random drift. Therefore, the correct answer is: B. how genetic drift can affect allele frequency in a population.
