Low thermal conductivity of heat insulating materials is due to its

ANS:B - high proportion of air space.

The low thermal conductivity of heat insulating materials is primarily due to their high proportion of air space. Air is a poor conductor of heat compared to solid materials. When heat insulating materials have a high proportion of air space, it reduces the material's ability to conduct heat effectively, thus resulting in lower thermal conductivity. Therefore, the correct option is: high proportion of air space.