Is a mode C transponder required in Class D airspace?

In Class D airspace, a mode C transponder is not required as a general rule, which leads to the answer being "no." The primary requirement for operating in Class D airspace is that the pilot must establish two-way communication with the air traffic control tower. While a mode C transponder may be required in other airspace classifications (such as Class B or when operating at certain altitudes in Class E airspace), in Class D, the key regulation does not mandate its use unless specified by the local air traffic control.

It's important for pilots to understand that while having a transponder can enhance safety and situational awareness, its absence in Class D airspace does not violate regulations as long as two-way communication is established with the tower.