Define an “axle” of a vehicle:

1. The vehicle’s back beam to which the vehicle’s spare wheel is connected.
2. A shaft which transmits the rotary motion from the gearbox to the engine.
3. Correct answer: Shaft or system of shafts running the width of the vehicle attached to the wheels.
4. The vehicle’s front part, designed to absorb the impact of low-speed collisions

WhyAn axle is a shaft or set of shafts across the vehicle that the wheels are mounted on. The bumper, spare wheel mount and drive shaft are other parts.