[C]
The reason why I say this, is because that is the only option that looks like the "flat shape." Notice that the other options have different designs that wouldn't look the same as the flat shape. The flat shape is just a rectangle you piece of cardboard with smaller rectangles at the end of each side. As you can see, shape C shows both smaller rectangles connected without any other design or cur out.
Hope This Helped! Good Luck!