The correct response is:
The wolves increased the population by eating beavers’ predators.
The reintroduction of wolves to Yellowstone helped control the populations of elk and other herbivores that were overgrazing the vegetation along streams and rivers. This allowed for the regeneration of willow and other plants, which in turn provided better habitats for beavers. With fewer predators of beavers in the area, their population was able to thrive as well.