* A. Phospholipid bilayer: This is the fundamental structure of the cell membrane. It forms a barrier between the inside and outside of the cell.

* B. Proteins: Proteins are embedded within the phospholipid bilayer and serve a multitude of functions:

* Transport: Moving molecules across the membrane.

* Receptors: Binding to signaling molecules.

* Enzymes: Catalyzing reactions.

* Structural support: Maintaining the shape and integrity of the membrane.

* C. Carbohydrates: These are often attached to proteins or lipids on the outer surface of the membrane. They play a role in:

* Cell recognition: Allowing cells to identify each other and interact.

* Cell signaling: Communicating with other cells.

* D. Cell wall: This is the key difference. Cell walls are rigid structures found outside the plasma membrane of plant cells, bacteria, fungi, and some protists. Animal cells do not have cell walls.

Therefore, the answer is D. cell wall.