The correct answer is: A. 8 km/h.
The maximum speed of the train while unloading ballast is 8 km/h. This is because the train needs to be able to stop safely if there is a problem with the unloading process. If the train were to travel at a higher speed, it would be more difficult to stop and could cause an accident.
Option B is incorrect because 4 km/h is too slow. The train needs to be able to travel at a reasonable speed in order to unload the ballast quickly and efficiently.
Option C is incorrect because 6 km/h is too fast. The train needs to be able to stop safely if there is a problem with the unloading process. If the train were to travel at a higher speed, it would be more difficult to stop and could cause an accident.
Option D is incorrect because 10 km/h is too fast. The train needs to be able to stop safely if there is a problem with the unloading process. If the train were to travel at a higher speed, it would be more difficult to stop and could cause an accident.