The hypothesis that best explains the effect of the brown-furred mutation on the rabbit population is:
The brown fur mutation was harmful because it made the rabbits more visible to predators, decreasing their survival.
This explanation aligns with the observation that the number of brown-furred rabbits decreases over generations, indicating that they are less likely to survive and reproduce in their snowy environment compared to their white-furred counterparts, who blend in better with their surroundings.