Explanation: RADAR receivers operate at microwave frequencies and require a stable local oscillator (LO) to mix with the incoming RF signal, converting it to a lower intermediate frequency (IF). * **Klystrons**, specifically reflex klystrons, are vacuum tubes designed to generate microwave frequencies and were historically common as local oscillators in microwave receivers, including early RADAR systems. * **Gunn diodes** are semiconductor devices that exhibit negative resistance and can oscillate directly at microwave frequencies. They are widely used as local oscillators in modern microwave receivers due to their small size, efficiency, and reliability. A **thyratron**, on the other hand, is a gas-filled tube primarily used as a high-power switch in pulse-forming networks, not as a stable microwave oscillator for receiver LO applications. Therefore, both klystrons and Gunn diodes are viable choices for RADAR receiver local oscillators.