Maximum heat dissipation occurs when thickness of insulation is

Question

For a current carrying wire of 20mm diameter and (k=0.5W/mK) exposed to air (h=20 W/m²K), maximum heat dissipation occurs when thickness of insulation is                                                                                  

      (a) 30mm                                                            (b) 25 mm

      (c) 20 mm                                                           (d) 15 mm