Answer:
E. This population is in Hardy-Weinberg equilibrium.
Step-by-step explanation:
A population in Hardy-Weinberg equilibrium maintains constant allele and genotype frequencies over generations. To obtain the Hardy-Weinberg equilibrium, none of the evolutionary forces (genetic drift, mutation, natural selection, etc.) should be operative on the gene pool as they deviate the population from the equilibrium.
According to the given information, the allele frequencies and hence the genotype frequencies in a population of butterflies are constant over two decades. This shows that the population is at Hardy-Weinberg equilibrium and is not evolving.