Question

Which keystone species feed on other animals and prevent them from damaging the ecosystem?

Answer 1

D.  Predators

Predators keep populations of other species in check.  This was pretty clearly demonstrated in Yellowstone National Park after the wolves were eradicated there.  The deer populations increased to the point that the aspen trees nearly disappeared due to the saplings' being eaten by deer.  Since the reintroduction of wolves to the park, the aspen trees have begun to flourish once again.