The correct answer is A. 15 kmph.
A loop line is a railway track that is connected to the main line at both ends. It is used to allow trains to pass each other or to stop at a station without interfering with the main line traffic.
The maximum speed on a loop line is 15 kmph. This is because the loop line is not as well-maintained as the main line and is therefore more likely to have defects. If a train were to travel at a higher speed, it could derail if it hit a defect in the track.
The other options are incorrect because they are higher than the maximum speed allowed on a loop line.