The correct answer is D. TD-912.
During power failure in automatic block section, the relief engine will proceed on the authority of the train dispatcher. The train dispatcher will issue a train order authorizing the relief engine to proceed. The train order will include the route that the relief engine must follow, the speed that the relief engine must travel, and the precautions that the relief engine must take.
Option A, TA-912, is the authority for a train to proceed in an automatic block section when the block signal is at stop. Option B, TB-912, is the authority for a train to proceed in an automatic block section when the block signal is at caution. Option C, TC-912, is the authority for a train to proceed in an automatic block section when the block signal is at all clear.
However, these options are not applicable during power failure in automatic block section. Therefore, the correct answer is D. TD-912.