The optimum thickness of enamel paint should be

Question

It is proposed to coat a 1mm diameter wire with enamel paint (k = 0.1 W/m-K) to increase heat transfer with air. If the air side heat transfer co-efficient is 1000 W/m²K, the optimum thickness of enamel paint should be                                

      (a) 0.25 mm                                                        (b) 0.5 mm

      (c) 1 mm                                                             (d) 2 mm