Explanation: The frequency of a klystron can be varied through different methods depending on the desired range of change. For **small changes**, adjusting the **repeller voltage** is the primary electronic tuning method, particularly in a reflex klystron. The repeller voltage controls the electron transit time within the resonant cavity. Varying this voltage alters how quickly the electrons are repelled back through the cavity, which in turn slightly shifts the oscillation frequency. For **large changes**, the physical dimensions of the klystron's resonant cavity (or cavities) must be altered. This is typically done **mechanically** by adjusting screws or plungers that deform the cavity, thereby changing its resonant frequency. Option A is incorrect because while anode voltage affects beam energy, it's not the primary or most effective method for tuning the oscillation frequency. Option B is nonsensical. Option C describes the modulation process, not a frequency tuning mechanism.