Which of the following is an example of an unbalanced external force?

Friction is an example of an unbalanced external force because it can cause an object to change its state of motion. When two surfaces interact, friction acts in the opposite direction to the movement of an object. If an object is pushed across a surface and friction opposes its motion, the net force acting on the object is not zero, leading to acceleration or deceleration depending on the magnitude of the friction compared to the applied force.

In contrast, gravity is a force that acts uniformly on objects, pulling them toward the center of the Earth, but it does not inherently cause a change in motion unless it is unopposed. Inertia is not a force but rather a property of matter that describes an object's resistance to changes in motion. Centripetal force is necessary for an object to move in a circular path but is balanced with other forces acting on the object, such as tension in a string or gravitational pull in the case of planetary motion. Therefore, friction is the only choice listed that can result in a net force acting on an object, leading to unbalanced conditions.