Under what condition are shipments containing dry ice prohibited?

The correct answer is that shipments containing dry ice are prohibited if the flight is conducted unpressurized. Dry ice releases carbon dioxide gas as it sublimates, which can lead to increased levels of carbon dioxide in the cabin of an unpressurized aircraft. In an environment where pressure is not maintained, this build-up of gas can create a hazardous situation for both crew and passengers. Therefore, regulations restrict the transport of dry ice in unpressurized conditions to ensure safety during flights.

The other options do not specifically address the unique hazards associated with transporting dry ice. The weight of the aircraft, the operational state of the auxiliary power unit (APU), and passenger count are not directly related to the dangers posed by dry ice but rather pertain to general operational limits or performance issues of the aircraft under different scenarios.