(c) Later, the car is moving forwards and the frictional forces suddenly increase to 2500N. The forwards force remains constant at 2000N.
Describe and explain what happens to the car.

Inelastic is the answer your looking for.

Answer(a)   the matererials that are in contact, the two materials and the nature of their surfaces/// (b)/// the force pushing the two surfaces together, pushing the surfaces together causes the more of the asperities to come together and increases the surface area in contact with each other