Final answer:
Doubling the speed of a car increases the force of impact by four times due to the squared relationship between speed and kinetic energy.
Step-by-step explanation:
When you double the speed of a car, the force of impact is four times greater. This is because the kinetic energy, which is proportional to the square of the speed, increases by a factor of four when the speed is doubled (as kinetic energy is given by the formula ½mv², where m is the mass and v is the velocity). A higher kinetic energy at the point of impact translates to a larger force of impact, leading to the answer being (a) Four times greater.