The answer is (c): well, sort of, as seen in an mpeg video by clicking your mouse on the photograph below. Maybe "(d) other" is a better answer. Perhaps you wish to take issue with the use of the term "centrifugal force."
If you turn the wineglass upside down over the olive, then move the glass around in circles, you can get the olive moving in circles around the inside of the wineglass bulge, where it is held up by its inertia. "Centrifugal force" is a bad term; the correct way to describe the situation is that you get the olive moving around in circles inside the wineglass such that its inertia holds it against the inside of the glass. You can then flip the wineglass over such that the olive remains in the glass, as demonstrated so eloquently by Gwen.
