Side BG is the diagonal through the interior of the cube
Explanation:
Given Cube with
Top face ABDC
Bottom Face EFHG
In a cube there are 4 interior diagonals
Side AD is the diagonal of the top face ABDC
Side CE is diagonal of the side face ACBE
Side DF is diagonal of the side face BDHF
Side BG is the diagonal through the interior of the cube
Hence , the Side BG is the diagonal through the interior of the cube