Volume of a cube = side × side × side. So volume = 56 × 56 × 56 = 175616 cubic mm. Cubic units are used for space inside a solid. Volume measures the space inside the cube, so we multiply length, breadth, and height; in a cube all three are the same side. That is why the shortcut is side³. The answer is in cubic units, so the correct choice is 175616 mm³.