The correct answer is A. Ecology.
Ecology is the scientific study of the relationships between organisms and their environment. It is a broad field that encompasses the study of the physical environment, the biotic environment, and the interactions between organisms and their environment.
Ecology is important because it helps us understand how the natural world works. It can also help us to manage natural resources and to protect endangered species.
The other options are incorrect because they do not accurately describe the study of the inter-relationship between organisms and their environment.
B. Ecosystem is a community of organisms interacting with their physical environment.
C. Phytogeography is the study of the geographical distribution of plants.
D. Ethology is the study of animal behavior.