The answer would be:
b. Diaphysis
Here is more about the diaphysis:
Long bones have 2 main parts. You have the diaphysis and the epiphysis. The diaphysis is the long hollow shaft that is made up of compact hard bone. The ends of the long bond, which are wider are the epiphysis.
Inside the long bone you have the medullary cavity and this is filled with yellow bone marrow and red blood cells are produced here.
