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 system to maintain a stable internal environment despite changes in the external environment. Modifying the external environment is not its definition. Controlling what enters and exits the cell is related to cell - membrane function. Movement of materials into the cell is just a part of cellular activity. The key concept of homeostasis is maintaining a stable internal environment.

Answer:

maintaining a stable internal environment