To identify the very good insulator among the given options, we need to consider the properties of each material and their ability to resist the flow of heat.


Sawdust is a good insulator due to its low thermal conductivity. The air trapped between the wood particles reduces heat transfer. However, it is not as effective as some other materials listed.


Cork is a natural material that is commonly used as an insulator. It has a low thermal conductivity and can effectively resist the flow of heat. Cork is often used in building insulation, soundproofing, and in the manufacturing of gaskets and seals.


Asbestos was previously used as an insulating material due to its high resistance to heat. However, it is now known to be a hazardous material that can cause serious health issues when inhaled. Asbestos is no longer recommended or used as an insulator.


Glass wool is made from recycled glass and is commonly used as insulation material. It has a high resistance to heat transfer due to its low thermal conductivity. The structure of glass wool consists of small air pockets that trap heat and prevent its movement. This makes glass wool a very good insulator.


Among the given options, glass wool is the best insulator. It has a high resistance to heat transfer and is commonly used in various applications where insulation is required. It is important to note that asbestos, although previously used as an insulator, is now recognized as a dangerous material and should not be used.