b. Fortunato has never been found.
The key sentence for the answer is "For half of a century no mortal has disturbed them." Half a century is fifty years. We know that Montressor built the wall that night and walled in Fortunato. This eliminates A. Also, C is just wrong since he entombed Fortunato, not the old bones. Option D is also wrong since Fortunato is dead - he's not sleeping. The statement "no mortal has disturbed them" best indicates that Fortunato has never been found.