The correct answer is: D. None of these.
The speed at which a train can pass after a rail failure is rectified depends on a number of factors, including the severity of the failure, the type of train, and the condition of the track. In general, trains will be able to pass at a higher speed after a rail failure if the failure is minor, the train is light, and the track is in good condition. However, it is important to consult with a railway engineer to determine the safe speed for a particular situation.
Option A (20 kmph) is too slow for most trains. Option B (25 kmph) is a more reasonable speed, but it may still be too slow for some trains. Option C (30 kmph) is a safe speed for most trains, but it may not be safe for all trains. Therefore, the correct answer is D. None of these.