Which one of the following elements is used as a timekeeper in atomic

Which one of the following elements is used as a timekeeper in atomic clocks?

Potassium
Caesium
Calcium
Magnesium
This question was previously asked in
UPSC CDS-1 – 2018
Atomic clocks utilize the precise frequency of electromagnetic radiation absorbed or emitted by atoms during an electron transition between energy levels. Caesium-133 atoms are most commonly used as the primary standard for defining the second in modern atomic clocks. The official SI definition of the second is based on the frequency of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Caesium-133 atom.
Caesium-133 is the standard element used as a timekeeper in traditional atomic clocks due to the stable and precisely known frequency of its atomic transition.
While Caesium clocks are the most widespread standard, other elements like Rubidium are also used, and advanced research is ongoing using elements like Strontium and Ytterbium in optical lattice clocks which offer even higher precision.