The percentage of each allele stayed constant over 50 generations. Hence the correct option is a.
In the scenario described, the severe drought and subsequent reduction in the butterfly population, specifically the elimination of all yellow individuals (aa genotype), result in a population with only individuals of the AA and Aa genotypes. With the given instructions to set the number of individuals with the AA genotype to 100 and the number of individuals with the Aa genotype to 100, the simulation essentially begins with a fixed population size.
According to the conditions for the Hardy-Weinberg equilibrium, one of the requirements is a large population. In this case, the initial setup does not adhere to the condition of a large population, as the total number of individuals is only 200. This deviation from the ideal scenario can lead to fluctuations in allele frequencies due to genetic drift, which becomes more pronounced in smaller populations.
Therefore, over the course of 50 generations, the percentage of each allele is likely to fluctuate rather than remain constant, as genetic drift exerts a more significant impact in smaller populations. The option stating "The percentage of each allele stayed constant over 50 generations" is not aligned with the principles of the Hardy-Weinberg equilibrium, given the small population size and the effects of genetic drift in this simulated scenario. Hence the correct option is a.