The correct answer is A. 15 kmph.
The leading cab of an electric engine is the one that is at the front of the train. It is responsible for controlling the speed and direction of the train. If the leading cab is defective, the assistant driver can operate the train from the rear cab, but the maximum speed will be limited to 15 kmph. This is because the rear cab does not have the same level of control over the train as the leading cab.
Option B is incorrect because the maximum speed is not 30 kmph.
Option C is incorrect because the maximum speed is not 40 kmph.
Option D is incorrect because the maximum speed is not normal. The maximum speed is limited to 15 kmph if the leading cab is defective.