Which one of the following correctly describes the principle of the working of an atomic clock?

Atomic clocks work on the principle of the extremely stable resonant frequency of electromagnetic radiation associated with electron transitions between two hyperfine energy levels of atoms such as cesium-133 (9,192,631,770 Hz) or rubidium. The SI second is in fact defined by the cesium-133 transition. Quartz crystals are used in ordinary watches, not in atomic clocks.