Make a claim about how changes in the sea otter population affect other organisms that share the same ecosystem

Sea otters are a keystone species, meaning their role in their environment has a greater effect than other species. ... By maintaining healthy kelp forests, sea otters also indirectly help to reduce levels of atmospheric carbon dioxide, a prevalent greenhouse gas, as kelp absorbs and sequesters carbon

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."