$407.$ Maximum heat dissipation occurs from a steel wire $($k $=0\mathrm{.}5$ W$/$m$.$ k$)$ of $15$ mm diameter exposed to air $($h $=20$W$/$m$2.$k$),$ when the insulation thickness is $\_\_\_\_\_\_\_\_\_\_$ mm$.$

$15$

$25$

$10$

$30$