Which of the following are used for accurately measuring very small time intervals?

Anatomicclockisaclockdevice thatusesanelectronictransitionfrequencyinthe microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary  standards  for  international  time distribution services, to control the wave frequencyoftelevisionbroadcasts,andinglobal navigation satellite systems such as GPS.