Which of the following is a technique for providing highly accurate and stable oscillators needed for microwave transmission and reception?
Why Microwave work needs frequency references good to parts per billion, and there are several proven ways to get there. GPS satellites carry atomic clocks, so a GPS-disciplined oscillator locks a local 10 MHz source to that timing signal for long-term accuracy. A rubidium reference is itself an atomic standard, excellent for both short and long term stability. A high-Q dielectric resonator held at constant temperature gives a very stable mechanical/electrical resonance directly at microwave frequencies, avoiding the drift of ordinary LC or crystal circuits.
Watch out Each single choice looks like the whole answer, but the question asks which technique is used, and all three are in common use in microwave stations and lab references.
GPS, rubidium, temperature-controlled dielectric resonator: three roads to microwave stability, all valid.