If the mountain lion population vanishes, the most likely outcome would be that the deer population will increase. Mountain lions are apex predators, and their absence would remove a significant predator from the ecosystem, allowing deer numbers to rise without the constraints of predation. This could lead to overgrazing of the willow trees, which could potentially have cascading effects on the entire ecosystem, including the trout population that relies on the willows for shelter. Thus, the correct response is:
The deer population will increase.