Explanation: Colpitts oscillators are widely favored in Variable Frequency Oscillators (VFOs) due to their excellent frequency stability and ease of tuning. The Colpitts design uses a voltage divider made of two capacitors in the tank circuit, with the active device (transistor or FET) connected between them. This capacitive feedback arrangement provides good isolation from load variations, contributing to better frequency stability across the tuning range. Tuning is typically achieved by varying a capacitor in the tank circuit, often a varactor diode, which allows for smooth frequency changes. While a Hartley oscillator (C) is also a common LC oscillator type using a tapped inductor for feedback, Colpitts often offers superior stability for VFO applications, especially when wide tuning ranges are desired. A Pierce oscillator (B) is primarily used with crystal resonators to achieve very high fixed-frequency stability and is not suitable for a variable frequency application. Negative feedback (D) is a principle used to stabilize amplifiers and reduce distortion; oscillators, by definition, rely on *positive* feedback to sustain oscillation.