The correct answer is A. 20 kmph.
A goods train without a brake van must reduce its speed to 20 kmph while passing a station if a tail lamp is not provided. This is because a tail lamp is used to indicate the end of the train to other trains and to people on the tracks. Without a tail lamp, there is a risk of a collision.
Option B is incorrect because 50 kmph is too fast for a goods train without a brake van to pass a station. Option C is incorrect because there is no such thing as a “normal” speed for a goods train passing a station. Option D is incorrect because a goods train is not required to stop at every station.