Concept:
Klystron tubes are microwave devices that use cavity resonators for oscillation.
Step 1: Types of klystrons
There are two main types:
(i) Two-cavity klystron (amplifier)
(ii) Reflex klystron (oscillator)
Step 2: Reflex klystron structure
It consists of only one cavity resonator.
Step 3: Working principle
Electrons are emitted, pass through the cavity, and are reflected back by a repeller electrode.
Step 4: Oscillation mechanism
The same cavity is used for both bunching and energy extraction.
Step 5: Conclusion
\[
\boxed{\text{One cavity resonator}}
\]
Final Answer:
\[
\boxed{(A)}
\]