The correct answer is option 3, Ernst Haeckel. Ernst Haeckel, a German biologist, first used the term "Ecology" in the year 1866. He derived the term from the Greek words "oikos" meaning "household" or "home," and "logos" meaning "study" or "science." Hence, the term "Ecology" is the scientific study of the relationships between organisms and their environment. Charles Darwin, option 1, is known for his theory of evolution and natural selection. Robert Whittaker, option 2, is known for his work in ecological niche theory. Arthur Tansley, option 4, is known for his contribution to ecosystem studies and coining the term "ecosystem."