Answer:
Option: D. Companies replaced hand-operated brakes with air brakes on trains.
Step-by-step explanation:
In the early period, trains manually stopped by involving in hand-operated breaks. The handbrake worked by handling large wheel in the coach, attached to chains and levers which set brake pads under the train. To improve the safety of the rail coach, air breaks replaced because they were much more powerful and reliable, than the manual one.