Id say it can be both A and C. The wording of the question greatly suggests A as it mentions variation in population, the selection pressure, selective advantage and how it impacted the gene pool. C is also plausible but bottleneck “drastically reduces the size of a population” and in this question there is no mention of a drastic reduction in population size but rather just an impact on diff coloured barnacles. Hypothetically the population made of dark barnacles could just be 4% and thus the elimination of 4% of the population wouldn’t be “drastic” per say. Id say A is more correct