Based on the information that the cell is round and incapable of producing its own food, the most appropriate organelle to assume the cell must have is:
C. plasma membrane
The plasma membrane is essential for all cells as it controls the movement of substances in and out of the cell. The other options (central vacuole, chloroplast, and cell wall) are associated with specific functions or types of cells (like plant cells), but the plasma membrane is a universal feature of all cells.