In the context of ecosystems, a keystone species is one that has a disproportionately large impact on its environment relative to its abundance. Among the options provided, the sea otter is often cited as a keystone species, especially in coastal marine ecosystems. Sea otters help maintain the balance of kelp forest ecosystems by preying on sea urchins, which, if unchecked, can overgraze kelp.
Therefore, the correct response is:
Sea otter