Copying sector by sector at the bit level provides the capability to examine slack space, undeleted clusters and possibly, deleted files.
With answer a, only the system files are copied and the other information recovered in answer b would not be captured.
Answer "Copying the disk directory" does not capture the data on the disk, and answer "Copying and authenticating the system files " has the same problem as answer "Copying the system files". Actually, authenticating the system files is another step in the computer forensics process wherein a message digest is generated for all system directories and files to be able to validate the integrity of the information at a later time. This authentication should be conducted using a backup copy of the disk and not the original to avoid modifying information on the original. For review purposes, computer forensics is the collecting of information from and about computer systems that is admissible in a court of law.