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 process by which organisms maintain a stable internal environment despite changes in the external environment. It is not about modifying the external environment (first option). Controlling what enters and exits the cell (third option) is a function of the cell membrane. Movement of materials into the cell (fourth option) is just one aspect of cellular processes. The key concept of homeostasis is maintaining a stable internal environment.

Answer:

maintaining a stable internal environment