Final answer:
The adaptation most likely occurred in the mouse population through natural selection, as the light-colored mice developed darker fur that provided them with a survival advantage in their environment.
Step-by-step explanation:
The adaptation most likely occurred in the mouse population through natural selection (option C). Natural selection is the process by which certain traits become more or less common in a population depending on their fitness in the environment. In this case, the light-colored mice developed darker fur through natural selection, as the darker fur provided them with a survival advantage in their environment. The mice with darker fur were able to better blend in with the darkened forest floor, making them less visible to predators and increasing their chances of survival and reproduction.