If the cell is round and known to be incapable of producing its own food, the organelle that you would assume the cell must have is:
B. plasma membrane
The plasma membrane is essential for all cells as it regulates the movement of substances in and out of the cell, regardless of their ability to produce food.
Chloroplasts (A) are involved in photosynthesis and would not be present in a cell that cannot produce its own food. A cell wall (C) is found in plant cells and some prokaryotes, but it does not apply to all cells. A central vacuole (D) is typically found in plant cells and is involved in storage and maintaining turgor pressure, but it is not a requirement for all cells either.