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 = ωn
The natural frequency of a given spring-mass system is:
\(M\ddot x + kx = F\cos \left( {\omega t} \right)\)
\(\omega={\omega _n} = \sqrt {\frac{k}{M}} \)