Answer: The force of reaction force of the ground accelerates the car on a level road.
Step-by-step explanation:
When the wheels apply tractive force on the wheels by applying torque that is generated by engine of the car, the wheels in contact with the road tend to move backwards at the point of contact thus exerting a force in the direction opposite to the motion of the car. Now according to Newton's third law an equal and opposite forces act on the car which tend to accelerate the car in the direction of motion.