The correct answer is: A cube on a smooth horizontal surface cannot be in stable equilibrium.
A cube can be in neutral equilibrium or unstable equilibrium, but not in stable equilibrium.
In neutral equilibrium, a cube will return to its original position after being disturbed. For example, if you tip a cube slightly to one side, it will eventually come to rest back on its original flat surface.
In unstable equilibrium, a cube will not return to its original position after being disturbed. For example, if you tip a cube slightly to one side, it will continue to tip until it comes to rest on one of its edges or vertices.
A cube cannot be in stable equilibrium on a smooth horizontal surface because there is no force that will return the cube to its original position after being disturbed. If you tip a cube slightly to one side, it will continue to tip until it comes to rest on one of its edges or vertices.