Final answer:
The normal force is what prevents the cup from falling to the ground when placed on a table, counteracting the gravitational force and allowing the cup to remain stationary.
Step-by-step explanation:
The force that prevents a cup from falling to the ground when placed on a table is the normal force. This force acts perpendicular to the surface of the table and supports the weight of the cup. According to Newton's second law, when an object is not accelerating, the net external force is zero. Thus, the normal force exerted by the table exactly balances the gravitational force (weight), and they are equal in magnitude but opposite in direction. When the cup is stationary on the table, the table deforms slightly under its weight until the normal force becomes equal to the weight of the cup, achieving equilibrium.