Is a Mode C transponder necessary to fly under the Class C shelf?

The appropriate answer is that a Mode C transponder is not required to operate under the Class C airspace shelf. While within Class C airspace, pilots are required to have a Mode C transponder, this rule does not extend to flying just beneath the Class C shelf. The airspace below the Class C does not impose the same transponder requirements, allowing pilots to navigate in that airspace without the necessity of having a Mode C transponder.

It's also important to remember that while certain operational rules apply within Class C airspace, the specific requirement for a transponder generally applies only to operations within the defined boundaries of the airspace, rather than underneath it. Consequently, a pilot operating below that shelf is not mandated to possess a Mode C transponder for compliance with aviation regulations.