Answer:
The correct answer is option b. "Genetic drift".
Step-by-step explanation:
Genetic drift is defined as a changes in a existing genotype among a population occurring just by chance, independently of a natural selection process. This changes are more evident when a population faces a devastating event like a disease, and only a few members of the population survive for being randomly resistant. Something similar is presented in the question, at which some moths survive to the chestnut blight fungus.