The correct answer is C) Two-second rule.
This means that a driver must maintain a distance that would take at least 2 seconds to cover, at the current speed of the car. This means that in case the car in front suddenly comes to a full stop, the driver of the car behind has 2 seconds react and hit his own brakes. A two-car length rule might be alright when driving at low speeds, but when going at high speeds (on a highway, for example), a two-car length can be covered in a fraction of a second, which means a sudden braking by the car in front would almost guarantee an accident.