Explanation: The primary operating frequency of a reflex klystron, like all cavity resonators, is fundamentally determined by the **physical dimensions of its resonant cavity**. The cavity acts as a tuned circuit, and its natural resonant frequency is directly dependent on its size and shape, much like the length of an antenna determines its resonant frequency. While the voltage applied to the repeller plate (D) can provide a narrow range of **fine-tuning** for the operating frequency by altering the electron transit time, it does not set the primary, fundamental frequency. The repeller voltage allows for slight frequency adjustment or modulation around the cavity's natural resonance. Options (B) and (C) are incorrect as they do not directly control the fundamental resonant frequency of the cavity. Klystrons do not typically use a "control grid" like lower-frequency tubes to set frequency, and the voltage on cavity grids relates more to electron acceleration or beam formation than the cavity's inherent resonant properties.