Answer:
B
Step-by-step explanation:
Any object will be acted on by a force, when that object is not moving (or has an acceleration equal to zero), then the forces acting on that object are considered to be balanced.
A: Is balanced, because a bridge has multiple structures in place that prevent it from being unbalanced, and falling over.
B: Is unbalanced, because during a roller coaster ride, the acceleration changes constantly. Hence, during its first drop, the roller coaster is being acted on by unbalanced forces.
C: Is balanced, because although there are two forces acting in opposite directions (the pulling from both sides), the overall net force is equal to zero.
D: Is balanced, because the car is not moving, and hence it will have zero acceleration.