The correct answer is A. Fall.
When an alternator connected to the bus-bar is shut down, the bus-bar voltage will fall. This is because the alternator is no longer generating electricity, and the bus-bar is now relying on other sources of power to maintain its voltage. If these other sources of power are not sufficient, the bus-bar voltage will fall.
Option B is incorrect because the bus-bar voltage will not rise when an alternator connected to the bus-bar is shut down. This is because the alternator is no longer generating electricity, and the bus-bar is now relying on other sources of power to maintain its voltage. If these other sources of power are not sufficient, the bus-bar voltage will fall.
Option C is incorrect because the bus-bar voltage will not remain unchanged when an alternator connected to the bus-bar is shut down. This is because the alternator is no longer generating electricity, and the bus-bar is now relying on other sources of power to maintain its voltage. If these other sources of power are not sufficient, the bus-bar voltage will fall.
Option D is incorrect because the bus-bar voltage will fall when an alternator connected to the bus-bar is shut down.