Final answer:
Illustration 4 shows the largest one-celled organism.
Step-by-step explanation:
The illustration that shows the largest one-celled organism is illustration 4. This is because the size of a cell can be determined by the magnification used in the microscope. If the magnification is higher, the cell appears larger in the field of view. Therefore, since illustration 4 has the highest magnification, the one-celled organism appears the largest.