Whoops -- I had jumped directly from the subject line to the image, so I missed the six cubes.
These are side/top/bottom views of an object constructed by joining six identical cubes together. Also, I suspect that the five given views are not necessarily oriented in any way.
I think that a rectangle signifies two adjacent cubes located the same distance from the viewer. In other words, the faces of these two cubes form a rectangle because each face lies in the same plane. (There could be another cube behind the rectangle, in a view.) But a square next to a rectangle indicates that the face we see as the square lies at a different distance (closer or farther) than the cubes forming the rectangle.
Looking at #5, there are three cubes whose faces are at the same distance, and there are two other cubes whose faces are at a different distance. Where is the sixth cube? I think I can imagine its position, to render the other given viewpoints.
If I'm correct, the missing view will look like an L-shape, with a square attached to one of its longer sides. :cool: