The question asks which of the given substances has the least thermal conductivity.
Thermal conductivity is a measure of a material's ability to conduct heat. Generally, materials conduct heat in the following order:
- Metals (highest conductivity)
- Non-metallic solids
- Liquids
- Gases (lowest conductivity)
Let's compare the options:
- A) Water is a liquid.
- B) Air is a gas.
- C) Iron is a metal.
- D) Glass is a non-metallic solid.
Based on the general order, gases have the lowest thermal conductivity. Among the given options, air is a gas.
Therefore, air has the least thermal conductivity.
The correct option is B.
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