At the starting station, passenger train must have ________ % of brake power. A. 75 B. 50 C. 90 D. 100

The correct answer is D. 100%.

According to the Federal Railroad Administration (FRA), passenger trains must have 100% of brake power at the starting station. This is to ensure that the train can stop safely in the event of an emergency.

Option A, 75%, is incorrect because it is not enough brake power to stop the train safely in an emergency. Option B, 50%, is also incorrect for the same reason. Option C, 90%, is incorrect because it is not the minimum required brake power according to the FRA.

Therefore, the correct answer is D. 100%.