The correct answer of this question is : Low thermal conductivity of hot coals.
EXPLANATION :
Before going to answer of this question, first we have to understand the nature of hot coals of wood in terms of conductivity.
The thermal conductivity of the wood is very small i.e it is thermally insulator.
Due to the less thermal conductivity of wood, the heat partially flows to the foot. The person will feel the heat, but not as much as the heat given by thermal conductors.
Hence, a barefoot person person can walk on hot coals of wood.