The correct answer is D: Colpitts, Hartley, and Pierce. Three common oscillator circuits are the Colpitts, Hartley, and Pierce oscillators. These are fundamental oscillator topologies used in radio frequency circuits. The Colpitts oscillator uses a capacitive voltage divider for feedback, the Hartley oscillator uses a tapped inductor (autotransformer) for feedback, and the Pierce oscillator uses a crystal for frequency control. These three oscillators are among the most commonly used in amateur radio equipment. They're used in transmitters, receivers, and frequency synthesizers to generate stable RF signals. Each has different characteristics that make them suitable for different applications.