Answer:
Keystone species.
Step-by-step explanation:
Keystone species may be defined as the species on which the other species depend directly or indirectly. The removal of keystone specie from the ecosystem can destroy the structure and functioning of the ecosystem.
Sea otters are predators of the sea urchin and regulate its population and forms kelp beds. Removal of sea otters from the ecosystem destroys the population of spawning fish and increase population of sea urchin destroys the kelp beds. Hence, sea otters acts as a keystone species because other species depends on the sea otter.
Thus, the correct answer is (C).