A simple mass-spring oscillatory system consists of a mass $m$, suspended from a spring of stiffness $k$. Considering $z$ as the displacement of the system at any time $t$, the equation of motion for the free vibration of the system is $m \ddot{z} + kz = 0$. The natural frequency of the system is

- $\dfrac{k}{m} \\$
- $\sqrt{ \dfrac{k}{m}} \\$
- $\dfrac{m}{k}\\$
- $\sqrt{ \dfrac{m}{k}}$