The correct answer is b) an unbalanced force. When an object is at rest or moving with a constant velocity, it means that the forces acting on it are balanced. However, when an unbalanced force is applied to the object, it causes a change in its motion. To determine the cause of the change in an object's motion, you need to analyze all the forces acting on it. If the net force acting on the object is zero (balanced forces), its motion will remain unchanged. But if the net force is nonzero (unbalanced forces), it will accelerate or decelerate in the direction of the force. So, to identify what causes a change in an object's motion, you need to examine whether the forces acting on it are balanced or unbalanced.
