If the cell is known to be round and incapable of producing its own food, the organelle you would assume the cell must have is the plasma membrane.
Here's a brief explanation for each option:
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Chloroplast: This organelle is responsible for photosynthesis and would be present in photosynthetic cells, such as plant cells, which produce their own food.
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Central vacuole: While many plant cells have a central vacuole for storage and maintaining turgor pressure, it is not a defining characteristic of all round cells, especially if they cannot produce their own food.
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Cell wall: This structure is common in plant cells and certain other types of cells, but not all round cells have a cell wall (many animal cells do not).
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Plasma membrane: This is a fundamental component of all cells, including round cells. It regulates what enters and exits the cell and provides structural support.
Thus, the best choice here would be the plasma membrane.