Final answer:
Fir is not considered a hardwood; it is a softwood, while oak, ash, and mahogany are typical hardwoods known for their durability and are commonly used for making furniture and flooring. The correct option is b.
Step-by-step explanation:
The wood that is not considered a hardwood among the options given is b. fir. Hardwoods typically come from deciduous trees which lose their leaves annually, and they usually have a more complex structure than softwoods, which come from evergreen conifers.
Fir is a type of softwood and hence, does not belong to the hardwood category. Hardwoods like oak, ash, and mahogany are known for their density and durability, making them popular choices for furniture and flooring.