Final answer:
The difficulty in pinpointing the exact number of Jack the Ripper's victims lies in the lack of evidence and poor record-keeping of the time, not a cover-up, meticulous record-keeping by the killer, or a high volume of unrelated crimes.
Step-by-step explanation:
It is hard to determine the precise number of murders associated with Jack the Ripper primarily due to lack of evidence and records from that era. At the time of Jack the Ripper's crimes, forensic science was in its infancy, and the killer left few clues that could conclusively link them to specific murders. This, combined with the fact that the area where the murders occurred was experiencing a high volume of violent crime, makes it challenging to ascertain which murders were the work of the same individual. Moreover, contemporary records were not as detailed or consistently kept as they are now, leading to discrepancies and uncertainty about the total number of victims. Therefore, option 'a' is correct in stating that the difficulty comes from a lack of evidence and records from that period.