The right option is; C. The keystone species is critical to maintaining the stability of an ecosystem, and changes in its population indicate changes in ecosystem health.
Keystone species are the species such as jaguar that has a superfluous impact on its environment relative to its population. The keystone species are important for the general function and structure of an ecosystem and they have an influence on the composition (in terms of plants and animals) of an ecosystem. It is useful for an ecologist to identify the keystone species in an ecosystem because the keystone species is important to maintaining the stability of an ecosystem, and any changes in the keystone species population shows that there is changes in ecosystem health.