Circle. The base of a cylinder is a circle and it is sliced parallel to the base.
If you slice a cylinder at an angle parallel to its base (the flat surfaces on both ends), then you will be left with two smaller cylinders with the same base but different heights. Another way to think of it is if you cross-section cut (cut it parallel to its base/end) a rolling pin in half, you now have essentially two shorter rolling pins. Hope this helps!
