Answer: b. keystone species
Step-by-step explanation:
The keystone species is the species which exerts an effect on the communities in which they grow. Such species helps in maintaining the diversity of local animal species.
The keystone species is the dominant predator species. It controls the population of the predator species.
According to the given situation, the seastar is the dominant predator species which is capable of controlling the population of the mussel species. Hence, the seastar could be a keystone species.