The correct answer would be, the wolves increased the beaver population by eating a competitor.
The reintroduction of wolves into Yellowstone National Park helped to regulate the populations of larger herbivores, like elk. With fewer elk and healthier populations of vegetation, particularly willows and aspens, the habitat became more suitable for beavers, which rely on these plants for food and building materials. As the landscape improved, beaver populations were able to thrive as a result of the changes brought about by the presence of wolves.