Answer: Zero.
Step-by-step explanation:
By the first Newton's law, we know that:
every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.
Now, we know that the car is moving with constant speed, then there is no net force acting on the car, which means that the car is already in equilibrium.
Then if we add one force to the situation, the car will not be anymore in equilibrium.
The correct option is zero.