The correct answer is: D. Ecosystem
An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the ecosystem through photosynthesis and is then transferred through the food chain. The abiotic components of an ecosystem include the air, water, soil, and climate. The biotic components include plants, animals, fungi, and bacteria.
Option A is incorrect because energy flow is a process that occurs within an ecosystem. It is the movement of energy from one organism to another.
Option B is incorrect because the environment is everything that surrounds an organism. It includes the physical environment, such as the air, water, and soil, as well as the biological environment, such as the plants and animals that live in the area.
Option C is incorrect because the atmosphere is the layer of gas that surrounds the Earth. It is made up of nitrogen, oxygen, argon, and other gases.