What allows the axle to move up and down without breaking the drive shaft?

The function of the universal joint is to allow for the rotational motion of the drive shaft while accommodating the up and down movement of the axle. In a vehicle, the axle must be able to move relative to the drive shaft, especially in response to changes in terrain or when the vehicle goes over bumps. The universal joint connects the drive shaft to the axle and permits this flexibility, ensuring that power can be transmitted smoothly even as the axle moves. This design prevents undue stress on the drive shaft, which could lead to mechanical failure.

The other options have different functions: shock absorbers help control the motion of the suspension, springs support the weight of the vehicle and provide cushioning, while ball joints connect the suspension components and allow for movement but do not directly facilitate the drive shaft's rotational movement in relation to the axle.