Wood being softer than coal is a physical property. Any chemical property has to deal with a change in somethings composition. When would is burnt, rotted, or not rusted, it is because there is some form of chemical reaction (Or lack thereof) inside of the wood. The property of being softer, however, does not come about by changing what wood is.
The physical property of wood is that it is softer than coal, which can be directly observed. The other options relate to chemical properties, as they involve changes in the composition of wood. Therefore, the answer is D.
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