A disc of mass m is attached to a spring of stiffness k as shown in the figure. The disc rolls without slipping on a horizontal surface. The natural frequency of vibration of the system is
(a) $\frac{1}{2\Pi }\sqrt{\frac{k}{m}}$
(b) $\frac{1}{2\Pi }\sqrt{\frac{2k}{m}}$
(c)$\frac{1}{2\Pi }\sqrt{\frac{2k}{3m}}$ 
(d)$\frac{1}{2\Pi }\sqrt{\frac{3k}{2m}}$ 