The correct answer is D: All these choices are correct. For highly accurate and stable oscillators needed for microwave transmission and reception, you can use a GPS signal reference, a rubidium stabilized reference oscillator, or a temperature-controlled high Q dielectric resonator. All three techniques provide the precision needed for microwave work. GPS references use the highly accurate atomic clocks in GPS satellites to discipline a local oscillator. Rubidium oscillators use atomic transitions in rubidium atoms for extremely stable frequency references. Temperature-controlled dielectric resonators provide stable, high-Q resonators that maintain frequency over temperature. All three are used in microwave systems where frequency accuracy and stability are critical. Each has different advantages - GPS provides absolute accuracy, rubidium provides long-term stability, and dielectric resonators provide good short-term stability.