Mention some repetitive phenomena in nature which could serve as time standards. Which one is most suitable?

Explanation

Some of the repetitive phenomena occurring in
nature are:
(i) Regular heart beat of a person,
(ii) Osccillations of a simple pendulum,
(iii) Vibration of atoms,
(iv) Rotation of earth about its axis.
Earlier, rotation of earth about its axis could be taken as most apporpriate. Now, we are shifting
to vibration of atoms as the most suitable
repetitive phenomenon.