Correct Answer - Option 4 :

\(\sqrt {\frac{k}{M}}\)
**Concept:**

When the natural frequency of the system coincides with the external forcing frequency, it is called resonance, that is, ω = ω_{n}

The natural frequency of a given spring-mass system is:

\(M\ddot x + kx = F\cos \left( {\omega t} \right)\)

\(\omega={\omega _n} = \sqrt {\frac{k}{M}} \)