A cube
There are two definitions for 'cuboid', but they both describe a shape with six flat surfaces (or 'faces').
Squares, rectangles, and the rhombus.
There can be no such shape. If there were a shape with n flat surfaces, then if you sliced off one vertex, you would have a shape with n+1 flat surfaces. So it is always possible to add one more flat surface.
A cuboid has got six flat surfaces.
A sphere
a rectangle
A cube has 6 flat surfaces. Most boxes of all shapes also have six flat surfaces
A cube has six flat surfaces, but so do other solids. Look up 'hexahedron' in wikipedia.
Six faces are flat.
cube There are a great many solid shapes that have only flat surfaces.
a cylinder
It is a cube or a cuboid