Answer:
b. keystone.
Step-by-step explanation:
A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance. Such species are described as playing a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.
Here are some examples of keystone species: Bison, prairie dog, and otter.
Hope it helps!