A small ring of radius r rolls without slipping in side a fixed hollow cylinder of radius R with small oscillations about stable equilibrium as shown in the fig. The frequency of oscillation is ?

a.1/2П (g/R-r)^0.5 b.1/2П(2g/R-r)^0.5 c.1/2П(3g/2(R-r))^0.5 d.1/2П(g/2(R-r))^0.5