The natural frequency of vibration of the mass less rigid bar connected to a spring

Question

Figure shows a single degree of freedom system. The system consists of a mass less rigid bar OP hinged at O and a mass m at end P. The natural frequency of vibration of the system is

(a) $f_n=\frac{1}{2\Pi }\sqrt{\frac{k}{4m}}$                                                    (b)$f_n=\frac{1}{2\Pi }\sqrt{\frac{k}{2m}}$

(c)$f_n=\frac{1}{2\Pi }\sqrt{\frac{k}{m}}$                                                    (d)$f_n=\frac{1}{2\Pi }\sqrt{\frac{2k}{m}}$