The critical thickness of the insulation of a copper pipe

Question

A copper pipe carrying refrigerant at - 200C is covered by cylindrical insulation of

thermal conductivity 0.5 W/mK. The surface heat transfer coefficient over the insulation is 50 W/m2 K. The

critical thickness of the insulation would be

 (a) 0.01 m   (b) 0.02 m   (c) 0.1 m  (d) 0.15 m