Which airspace does NOT require a transponder?

Class D airspace typically requires pilots and their aircraft to have a two-way radio communication capability with air traffic control, but it does not specifically mandate the use of a transponder. In contrast, Class B and Class C airspaces do require aircraft to be equipped with a transponder that meets certain performance specifications, facilitating better radar tracking and communication with air traffic control.

Class E airspace has varied requirements depending on the specific situation and altitude; however, in many cases, it also does not require a transponder unless the aircraft is flying above certain altitudes or in specific conditions, such as when transitioning to higher airspace where transponder use becomes mandated.

Therefore, out of these classifications, Class D stands out as the category that does not impose a transponder requirement, although it still necessitates communication protocols with ATC.