Which of the following codes indicates an aircraft that has a transponder with Mode C?

The code that indicates an aircraft equipped with a transponder that has Mode C is represented by the symbol "/C." However, the options provided include different representations. The correct choice is the one that signifies that the aircraft is equipped with a transponder capable of reporting altitude, which is a primary function of Mode C.

The code "/U" is used to indicate that the aircraft is equipped with a transponder but does not specifically denote Mode C. Instead, it is used for transponders that may have advanced capabilities beyond what is required for basic air traffic control communication.

In contrast, the other choices represent different capabilities of transponders. For example, "/X" typically suggests the aircraft has a transponder but lacks precise capabilities.

Thus, the choice of "/U" signifies that the aircraft is capable of transponder operations, which includes Mode C capabilities, allowing ATC to receive both position and altitude data from the aircraft.