Answer:
Friction
Step-by-step explanation:
This is because friction tends to oppose the motion of an object. Since the unbalance force or net force causes a motion of the object, the frictional force would tend to oppose the object until it is large enough to balance the net force. When this is done, equilibrium is achieved.
At this instance, there is no net force acting on the object.
So, the forces on the object become balanced and in a state of equilibrium.