So, I'm doing Exercise 5.
Here is what I think about a).
So, we consider the first rectangle on the plane x y and the second on the plane x z. We take a point of the first rectangle and one of the second rectangle with the same coordinates. Proceeding like that, we obtain 'the diagonal of the parallelepiped.' So, I think it is more or less the correct answer. However, I cannot justify that.
If I am right, then b) is quite the same, but we have 'the diagonal plane of a three-dimensional coordinate system.'
I have no idea what to do with e). It seems impossible.