Show that a light ray that strikes two mutually perpendicular reflecting surfaces is reflected back exactly parallel to the direction from which it came.

Take a look at the attachment below. As you can see, the figure demonstrates a 90 degree angle such that the surfaces are perpendicular, and hence are independent of the angle of incidence. You could say that this forms a part of a parallelogram -

hi there!

out of the two, (b) precision would be more reasonable.   however, both a and b could be pretty reasonable.

-awesomerepublic   : )