The correct answer is: A. Back
The advanced starter and IBS are located at the back of the train. If they are defective, the SM shall issue T/369(3b) authority with private number of the back station to pass both advanced starter and IBS. This is to ensure the safety of the train and its passengers.
The other options are incorrect because they do not refer to the back station.
Option B: Next. The next station is the station after the back station. If the advanced starter and IBS are defective, the SM shall not issue T/369(3b) authority with private number of the next station to pass both advanced starter and IBS. This is because the next station may not be able to provide the necessary assistance to the train.
Option C: Both. The SM shall not issue T/369(3b) authority with private number of both the back station and the next station to pass both advanced starter and IBS. This is because the SM needs to issue T/369(3b) authority with private number of only one station.
Option D: None of these. None of the options are correct.