The correct answer is option 2 - Caesium. Atomic clocks are devices that measure time based on the oscillations of atoms. In particular, caesium atomic clocks are used as timekeepers.

Caesium is used because it has a very precise natural frequency of oscillation. In atomic clocks, caesium atoms are excited to a specific energy level and then allowed to return to their ground state. The time it takes for the caesium atoms to complete this cycle is used as a reference for measuring time.

It is important to note that other elements such as potassium, calcium, and magnesium are not typically used as timekeepers in atomic clocks. While they may have other valuable uses and applications, their natural frequencies of oscillation are not as precise as caesium.

In summary, option 2 - Caesium is the correct answer because it is specifically used as a timekeeper in atomic clocks due to its precise natural frequency of oscillation.