Answer:
The answer is a. Keystone species is the sea otter. Sea otters, playing a critical role in containing the urchin populations, prey on urchins and thus control the numbers of kelp grazers to maintain the forest.
Step-by-step explanation:
A keystone species is the top consumer or an animal referred to be at the top of the food chain. Keystone species are mostly carnivorous secondary or tertiary consumers that maintain the balance of an ecosystem.
* Please refer to the attached image of Yellowstone's gray wolves . It is one of the most remarkable examples of a keystone species regulating an entire ecosystem.
In this scenario, the kelp are the producers i.e. they provide food, shelter for herbivores and the first consumer i.e. the sea urchin. The secondary consumer i.e. the seas otter feeds on the sea urchin and keeps their population in check. This helps sustain am equilibrium between kelp forests and sea urchin population.