Maximum heat dissipation occurs when thickness of insulation is

Question

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

      (a) 30 mm                                                           (b) 25 mm

      (c) 20 mm                                                           (d) 15 mm