Explanation: A three-circuit regenerative receiver, like any electronic device, requires all its constituent parts to function correctly. * **A) Tubes, power supply, and coils** are fundamental. Tubes provide amplification and detection, the power supply energizes the circuit, and coils form tuned circuits and provide feedback for regeneration. Any issue here prevents basic operation. * **B) Switches, pigtails, and variable capacitors** are critical for control and tuning. Poor switch contacts or broken pigtails interrupt signal paths, and a shorted variable capacitor disables tuning or regeneration control. * **C) Grid leak/capacitor, audio transformer, jacks/cords, and antenna/ground** are essential for detector biasing, audio output, and signal input. Problems with any of these will prevent proper reception or audio. Troubleshooting a receiver involves checking all potential failure points, from power input to signal output. Therefore, checking "All of the above" components provides a comprehensive approach to diagnose issues in a regenerative receiver.