If the cell is round and incapable of producing its own food, the most appropriate organelle it must have is:
A plasma membrane
The plasma membrane is essential for all cells, as it serves as the boundary that separates the interior of the cell from the external environment. Chloroplasts are involved in photosynthesis and are not found in cells that cannot produce their own food. A cell wall is typically found in plant cells and some fungi, and a central vacuole is usually found in plant cells, but neither is required for a round cell that cannot produce food.