Which is most likely a keystone species?

Question

a
wolves in Yellowstone National Park, which eat elk, who graze on willows, on which beavers depend to build their dams
b
feral cats, which hunt a wide variety of animals, including birds, lizards, rodents, and insects
c
deer, which eat a variety of plants, including shrubs and young trees, and are consumed by large predators like mountain lions, wolves, and bears
d
cane toads in Australia, which were introduced to eat cane beetles, but eat anything and survive in various habitats

Answer 1

a. Wolves in Yellowstone National Park, which eat elk, who graze on willows, on which beavers depend to build their dams