Answer:
A. The force of gravity on the car.
Step-by-step explanation:
A free-body diagram of an object must include all the forces acting on the object itself. Let's analyze each statement separately.
A. The force of gravity on the car. --> This is one of the forces acting on the car, so it must be included in the free-body diagram
B. The force of the engine in turning the driveshaft of the car. --> This force does not act on the car itself, so it should not be included in the diagram
C. The horizontal component of force that the tire exerts on the road. --> This force is exerted on the road, not on the car, so it should not be included
D. The velocity vector for the moving car. --> Velocity is not a force, so it should not be included