$${1 \over {r^2}}\,{\partial \over {\partial r}}\left( {r{{\partial T} \over {\partial r}}} \right) + {q \over k} = {1 \over \alpha }\,{{\partial T} \over {\partial t}}$$

$${1 \over {r^2}}\,{\partial \over {\partial r}}\left( {{r^2}{{\partial T} \over {\partial r}}} \right) + {q \over k} = {1 \over \alpha }\,{{\partial T} \over {\partial t}}$$

$${{{\partial ^2}T} \over {\partial {r^2}}} + {q \over k} = {1 \over \alpha }\,{{\partial T} \over {\partial t}}$$

$${{{\partial ^2}} \over {\partial {r^2}}}\left( {rT} \right) + {q \over k} = {1 \over \alpha }\,{{\partial T} \over {\partial t}}$$

$$a\,\,b\,\,c\,\,d$$

$$b\,\,c\,\,a\,\,d$$

$$d\,\,a\,\,b\,\,c$$

$$d\,\,c\,\,b\,\,a$$

Copper

Steel

Glass wool

Refractory brick

Material with higher thermal conductivity should be used for inner layer and one with lower thermal conductivity for the outer.

Material with lower thermal conductivity should be used for the inner layer and one with higher thermal conductivity for the outer.

It is immaterial in which sequence the insulating materials are used

It is not possible to judge unless numerical values of dimensions are given