The correct answer is: C. Arthur G Tansley.
Arthur Tansley was a British botanist who coined the term “ecosystem” in 1935. He defined an ecosystem as “the whole system (in the sense of physics) including not only the organisms but also the physical factors of the environment, acting as a complex interacting whole.”
Odum was an American ecologist who studied the structure and function of ecosystems. Clements was an American plant ecologist who studied plant communities. Elton was an English zoologist who studied animal populations.
All of these scientists made significant contributions to the field of ecology, but it was Tansley who coined the term “ecosystem” and gave it its modern meaning.