The correct answer is C. 50 meters.
The distance between the fire affected train and the rest of the train should be at least 50 meters to ensure the safety of passengers and crew. This is because a fire can spread quickly and easily in a train, and it is important to give firefighters enough space to work and to prevent the fire from spreading to other parts of the train.
Option A is incorrect because 25 meters is not enough distance to prevent the fire from spreading. Option B is incorrect because 45 meters is not enough distance to prevent the fire from spreading. Option D is incorrect because 100 meters is too much distance. It is important to remember that the distance between the fire affected train and the rest of the train should be enough to ensure the safety of passengers and crew, but it should also be short enough to allow firefighters to work effectively.