What are the four options for communication if DC is not available?

When considering communication options in aviation, particularly when direct communication (DC) is unavailable, the ability to connect with other Air Traffic Service (ATS) units is crucial. These other ATS units serve as a network that can maintain situational awareness and provide necessary support to aircraft in the airspace.

Communicating with other ATS units allows for the relay of vital flight information, coordination of traffic, and overall management of airspace safety. This ensures that even in the absence of direct communication, there is still a means to receive instructions, clearances, and updates regarding traffic and weather conditions that are essential for the safe operation of flights.

The other options, although they may play a role in certain situations, do not provide the same level of operational safety or control in air traffic management. Other aircraft would not have the authority or the complete situational overview to effectively manage air traffic, while airlines and other agencies may not have the role of real-time air traffic control coordination necessary in such scenarios.