Explanation: In a reflex klystron, electrons are accelerated towards a resonant cavity, pass through it, and are then reflected back by a negatively charged repeller electrode, passing through the cavity a second time. The repeller voltage directly controls the electric field in the drift space, which determines the electron transit time. For stable oscillations to occur, electrons must return to the cavity in the correct phase to reinforce the RF field. By slightly varying the repeller voltage, the electron transit time is altered, allowing for fine adjustment of the oscillation frequency within the cavity's resonant bandwidth. *Incorrect options:* A) Adjusting the flexible wall of the cavity changes the cavity's physical dimensions and thus its fundamental resonant frequency. This is a method for *coarse* mechanical tuning, not fine adjustments. C) An Automatic Frequency Control (AFC) system *uses* a tuning mechanism (like varying repeller voltage) to maintain frequency stability, but it is not the mechanism of adjustment itself. D) Varying the cavity grid potential typically influences the electron beam's characteristics rather than serving as the primary fine-tuning method for frequency.