what will be the critical radius of insulation of a metallic cylinder?

Question

 A cylinder made of a metal of conductivity 40 W/mK is to be insulated with a material of conductivity 0.1 W/mK. If the convective heat transfer coefficient with the ambient atmosphere is 5 W/m²K, the critical radius of insulation is (IES-01)

(a) 2 cm                       (b) 4 cm

(c)8 cm                        (d) 50 cm