Final answer:
Mode C is the common military/civil mode for air traffic control use, providing altitude information which is critical for maintaining safe aircraft separation.
Step-by-step explanation:
The student's question refers to the different modes of transponder signals used in aviation for air traffic control identification and tracking. Mode A, Mode C, and Mode S are different types of transponder modes. The common military/civil mode for air traffic control use is Mode C. Mode C provides altitude information to air traffic controllers, which is essential for maintaining safe separation between aircraft. Meanwhile, Mode A provides identification but no altitude information, Mode S provides enhanced data capabilities including aircraft identity, altitude, and additional data, and Mode X is not a standard mode of operation for civilian air traffic control.