What does Mode A of a transponder do?

Mode A of a transponder specifically transmits an identifying code, also known as a squawk code. This code is assigned to an aircraft by air traffic control and allows radar systems to identify the specific aircraft on their screens. Each aircraft has a unique squawk code that helps air traffic controllers manage and track multiple aircraft in a given airspace, improving situational awareness and safety.

The other functions described in the other choices do not pertain to Mode A. For instance, altitude information transmitted by a transponder pertains to Mode C, while voice communication is not a function of any transponder mode and is generally accomplished through radio communication. Data exchange between aircraft, such as with ADS-B, involves different technology beyond the basic functions of Mode A. Thus, the defining characteristic of Mode A is its role in transmitting identifying codes for easy identification.