Final answer:
Balanced forces cause an object to either remain at rest or to move at a constant speed. If an object is not accelerating, it means that the net forces acting upon it are balanced. In an example of a car moving at a constant velocity, the net horizontal force is equal to the friction force, which is why the car does not accelerate.
Step-by-step explanation:
Balanced forces act on objects that are not moving or moving at a constant speed. Balanced forces mean that the net force acting on the object is zero. Therefore, if the object was initially at rest, it will remain at rest. If the object was moving, it will continue to move at a constant velocity without accelerating. For example, if a car is moving at a constant speed, the knowledge that it is not accelerating (constant velocity) indicates that the net horizontal force on the car is balanced with the friction force. On the other hand, if a car is accelerating, this tells us that there is an unbalanced force where the horizontal force exceeds the frictional force, resulting in a change in velocity.
In the scenario of an object on a smooth icy surface with zero friction, when the force applied to accelerate it stops, the object's acceleration becomes zero (option a), but it will continue to move at the last constant speed it achieved due to its inertia (Newton's first law of motion).