The correct answer is: C. 40 km/h.
The maximum speed of a tower wagon is 40 km/h. This is the speed at which the wagon can travel safely and efficiently. If the wagon is driven faster than this, it may become unstable and could tip over.
Option A: 25 km/h is too slow for a tower wagon. This speed would not be practical for transporting materials or people.
Option B: 20 km/h is a safe speed for a tower wagon. However, it is not the fastest speed that the wagon can travel.
Option C: 40 km/h is the maximum speed for a tower wagon. This is the speed at which the wagon can travel safely and efficiently.
Option D: 50 km/h is too fast for a tower wagon. This speed would not be practical for transporting materials or people, and it could cause the wagon to tip over.