Final answer:
A keystone species is one whose presence is vital for maintaining biodiversity and ecological community structure. Examples include the intertidal sea star and the banded tetra fish. The correct option is A.
Step-by-step explanation:
Keystone Species
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community's structure.
The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species.
When this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity.
Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community.
If these fish were to become extinct, the community would be greatly affected. The correct option is A.