The correct answer is Kinetic friction between the tire and the road.

• A car is moving at the sped of 40.0 km/hr.
  • The driver raises the engine and the car accelerates to a speed of 50.0 km/hr.
  • This acceleration is due to Kinetic friction between the tire and the road.
  • If you apply a little more force, the object "breaks free" and slides, although you still need to apply force to keep the object sliding.
    • This is kinetic friction. 

• Rolling resistance (for vehicles) is the force per unit vehicle weight required to move the vehicle on level ground at a constant slow speed where aerodynamic drag (air resistance) is insignificant and also where there is no traction (motor) forces or brakes applied.
  • In other words, the vehicle would be coasting if it were not for the force to maintain a constant speed.
• There are two forms of friction: Kinetic and Static.
  • If you try to slide two objects past each other, a small amount of force will result in no motion.
  • The force of friction is greater than the applied force.
    • This is static friction.
  • If you apply a little more force, the object "breaks free" and slides, although you still need to apply force to keep the object sliding is called kinetic friction.