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

Every mirror image flips the figure over. (c) (upright mirror), (d) (flat mirror) and (a) (slanting mirror) are all flipped versions. (b) is the boot turned half a turn in its own plane: the toe now points left and the top is down, but it is not flipped over, so it is no mirror image.