Final answer:
Southern sea otters are a keystone species in kelp forest ecosystems, playing a critical role in maintaining the ecosystem's structure and biodiversity.
Step-by-step explanation:
Southern sea otters are a keystone species in the ecosystems where they reside. The correct answer to the student's question is b. kelp forest. Kelp forests rely on the predation of sea otters on sea urchins to prevent the sea urchins from overgrazing and destroying the kelp. Without sea otters, the kelp forest ecosystem would be dramatically altered, which would affect the diversity and population of other species within the ecosystem. Just like how the removal of the intertidal sea star affects mussel populations and overall biodiversity, the sea otter has a similar influential role in maintaining the structural integrity and biodiversity of the kelp forest ecosystem.