If an unbalanced force were applied to the glass of water (for example, someone pushing the glass, lifting it up, etc.), this would cause it to move. However, a balanced force (if two people were to simultaneously pull the glass in opposite directions with the same amount of force) would not cause the glass to move. Gravitational force acts on all objects; the force due to gravity is already acting on the cup at rest and will not cause it to undergo motion. Friction is also already acting on the cup in the form of static friction, making it harder for someone to move the cup, so frictional force will not cause the cup to move either. Therefore, the correct answer is D. an unbalanced force.
