Final answer:
When trains are traveling between a speed of 101-125 mph, you should stand back at least 40 feet from the track for safety. Therefore, the correct answer to the question is: 40 feet
Step-by-step explanation:
When trains are traveling between a speed of 101-125 mph (161 - 200 km/h), you should stand back at least 40 feet from the track. This distance allows for the safety of both the train passengers and pedestrians. Standing too close to the track can be dangerous due to the high speed and potential flying debris.
Therefore, the correct answer to the question is: 40 feet