Explanation: A Gunn oscillator utilizes a Gunn diode, which exhibits negative resistance at microwave frequencies. To sustain oscillations, this diode must be coupled to a resonant structure. The resonant cavity provides this structure, acting as an electromagnetic resonator that determines the specific frequency at which the oscillator will operate. Its physical dimensions, such as length and diameter, dictate the resonant frequency. Therefore, changing the cavity's size or tuning it (e.g., with a screw) directly alters the output frequency. A) The power supply voltage biases the diode and affects output power, but not the fundamental frequency. B) The type of semiconductor (e.g., Gallium Arsenide) determines the diode's operational characteristics and frequency *range*, but not the specific, precise operating frequency. D) Loading by a mixer is an external factor that might affect stability or power, but it doesn't determine the oscillator's inherent frequency.