For both part A and part B, the cross-section would be a square.
For part c, the reasons the shapes are the same is the 3-D figure of a cube has 6 faces that are all squares. Since every face is a square, you cannot cut a cross-section that is any shape but a square. Cross-sections in 3-D prisms can reflect the face they are nearest to, so with every face being identical on a cube, the cross sections are also identical.
I hope this helps! :)
