The aerobic exercise known for its low-impact nature that involves a machine simulating a walking or running motion is called the elliptical trainer. However, since it wasn't listed among your options, of the options provided, the treadmill would be the closest choice, although it does involve a higher impact compared to an elliptical.
If you're specifically looking for the most low-impact option among common aerobic exercises that simulate walking or running, the elliptical is the best choice.