* Maintaining a stable external environment is primarily the function of an organism's organ systems (like the circulatory, respiratory, and excretory systems). Cells contribute to this overall homeostasis, but they don't directly maintain the stability of the external environment.

Let's look at the other options:

* B. Making macromolecules for an organism: Cells are the fundamental units of life and are responsible for synthesizing essential molecules like proteins, carbohydrates, lipids, and nucleic acids.

* C. Breaking down molecules: Cells use various processes like cellular respiration to break down molecules to release energy or recycle components.

* D. Removing waste from the cell: Cells have mechanisms to expel waste products generated during metabolism.

In summary: While cells contribute to the overall homeostasis of an organism, the primary function of maintaining a stable external environment belongs to organ systems.