Question

which best defines homeostasis?
modifying the external environment
maintaining a stable internal environment
controlling what enters and exits the cell
movement of materials into the cell

Explanation:

Homeostasis is the ability of an organism or cell to maintain a stable internal environment despite changes in the external environment. Modifying the external environment is not homeostasis. Controlling what enters and exits the cell is related to membrane - transport functions, and movement of materials into the cell is just a part of cellular processes. The key concept of homeostasis is maintaining a stable internal environment.

Answer:

maintaining a stable internal environment