Three panels show this flag in a mirror held along some line (upright, flat or slanting). Which panel can NOT be a mirror image?

A mirror always flips the picture over. (a), (c) and (b) are flips in an upright, a flat and a slanting mirror. (d) is the flag turned half a turn in its plane: the green flag points left at the bottom, but the picture has not been flipped over, so no mirror gives it.