The correct answer is D) automatic bus transfer device. When the normal power supply fails, the automatic bus transfer device automatically transfers the emergency bus to the emergency generator, allowing it to provide power to the essential systems. This is a standard safety feature required by Coast Guard regulations to ensure continuity of power in the event of a main power failure. The other options are incorrect because: A) line connection feeder is not involved in the automatic transfer process, B) power failure alarm bus does not directly connect the generator to the emergency bus, and C) main bus tie feeder is not responsible for the automatic transfer to the emergency generator.