A solid cube with edge length 2 is sliced by a plane passing through two opposite corners of the cube. This creates a cut surface, the shaded rhombus shown [below]. What is the perimeter of the shaded rhombus?
I've attempted to make an illustration, which you see below. Note that the illustration is in two parts: a drawing based on the above description, plus a "closeup" that will help us analyze the problem.
I'm of the opinion that the perimeter of the rhombus in question is 4√5. Segment AC is the hypotenuse of a right triangle whose legs measure, respectively, 2 (Segment AB) and 1 (Segment BC). We can check visually that all the other edges of the rhombus are the same length, so this rhombus is a square with perimeter 4√5.
I bet that, once MGRE's site is back in order, one of the answer choices will indeed be 4√5.
By the way, here's a horizontally squished version of what I currently see at the MGRE page:
As you see, there's no illustration, and only one visible answer choice.