Explanation:

The Sea Otter (Enhydra lutris) is a classic example of a keystone species. A keystone species is an organism that helps define an entire ecosystem; without its presence, the ecosystem would be dramatically different or cease to exist. Sea otters control the population of sea urchins, which in turn prevents overgrazing of kelp forests. The health of kelp forests, which provide habitat and food for many other species, depends on the presence of sea otters. Option (4) 'Keystone species' is correct. Option (1) 'Flagship species' are charismatic species that serve as a symbol for a conservation effort. Option (2) 'Umbrella species' are species whose conservation indirectly protects many other species within the same ecosystem. Option (3) 'Indicator species' are organisms whose presence, absence, or abundance reflects a specific environmental condition. While sea otters might exhibit some characteristics of these other categories, their primary ecological role is that of a keystone species.