The answer is C.
In terms of ecology, sustainability is a term used to describe the long-term survival or continuation of an ecosystem. There are certain environmental resources that are required by living systems. Resources such as water, oxygen, carbon, and nitrogen have a finite quantity on Earth. Because these resources are continually being renewed in a cycle, they can be used over and over.
For example, the water cycle ensures that water used by one organism will be available for another organism in the future.