If the cell is known to be round and incapable of producing its own food, it is likely to be a type of animal cell. In this case, the organelle that you can assume the cell must have is the plasma membrane.
Chloroplasts are organelles responsible for photosynthesis and are found in plant cells and some algae, which produce their own food. Since the cell is incapable of producing its own food, it would not have chloroplasts.
The central vacuole and cell wall are typically found in plant cells, and the cell wall is not present in animal cells. Therefore, the best choice is the plasma membrane.