(C) The elevator. Initially, on the 100th floor, you're standing on a stationary elevator floor with gravity pulling you down the elevator shaft. You experience the feeling of weight because you're being stopped by moving by the floor. However, when the elevator pulls down, your gravity of 9.81 m/s^2 is seemingly reduced because the elevator begins to pull away from you. If you have nothing to "feel" your weight against, you won't feel your weight. You'll also have the appearance of weightlessness because you'll be traveling at the same speed as the inside of the elevator, so you'll have no visual reference to look at and tell you that you're moving.
This is exactly how astronauts train to operate in zero gravity. There's a large 737 that has been specially outfitted with just one large cabin and more powerful engines than normal. This plane, the "Vomit Comet," (no, I'm not the one who made that name up) will climb to a very high altitude, with everyone experiencing gravity, then it will quickly nose over and free-fall toward the surface. Since the astronauts inside are falling just as fast as the plane, and there is no visual cue, and not even any moving air to give any sense of motion, it feels like zero-g, or "weightlessness." I'm sure you've seen some video of them flipping around, playing with blobs of water, and so on.
So dropping elevator... dropping airplane... same thing. You "feel weightless."