Explanation:

A Keystone species (A) is an organism that has a disproportionately large effect on its environment relative to its abundance, often defining the entire ecosystem structure and function. A Flagship species (B) is chosen to represent an environmental cause. An Umbrella species (C) is one whose conservation indirectly protects many other species. An Indicator species (D) reflects environmental conditions. Thus, (A) best describes an organism that defines an ecosystem.