Explanation:

- The study of the environment involves understanding the complex interrelationship between biotic (living things like plants, animals, and humans) and abiotic (non-living elements like water, air, and minerals) components.

- Option 2: Interrelationship

- This is the most appropriate term. It highlights how living and non-living elements interact and affect each other in ecological systems.

- Option 1: Sustainable development

- This term refers to development that meets present needs without compromising future generations' ability to meet theirs. It's related but not the same as the basic ecological focus.

- Option 3: Environment

- While environment is the overarching term for everything surrounding us, it doesn't specifically refer to the interaction between its components.

- Option 4: None of the above

- Not applicable, since there is a correct term provided in the options.