Question

here the car is dramatically slowed down while not gaining height. what caused the car to slow down?
kinetic energy becomes potential energy.
kinetic energy is destroyed by the brakes in the track.
kinetic energy is converted into thermal energy (heat) via brakes in the track.

Explanation:

According to the law of conservation of energy, energy cannot be destroyed. Since the car is not gaining height (so no conversion to gravitational potential energy), when it slows down (loses kinetic energy), the kinetic energy is converted into another form. Friction (like from brakes) causes kinetic energy to be converted into thermal energy (heat).

Answer:

Kinetic energy is converted into thermal energy (heat) via brakes in the track.