The correct answer is B. The wolves increased the population by eating beavers' predators.
The reintroduction of wolves to Yellowstone led to a decrease in the populations of elk, which were overgrazing willow and other vegetation critical for beaver habitats. With fewer elk around, the vegetation could recover, allowing beavers to flourish as they found more suitable materials for building dams. Thus, the wolves indirectly supported the beaver population by controlling the elk numbers, which are considered a competitor in the ecosystem for resources that beavers rely on.