Final answer:
All of the options listed contribute to making fraud more difficult to detect, including collusion, forgery, and off-book frauds.
Step-by-step explanation:
All of the options provided in the question make fraud more difficult to detect. Collusion with outsiders often involves multiple parties and may include complex schemes that are harder for auditors to uncover. Forgery, which can involve the falsification of documents, can be particularly challenging to identify, especially since GAAS (Generally Accepted Auditing Standards) auditors are not routinely trained in this area. As for off-book frauds, these are sophisticated because they do not leave a direct trail on the company's financial records, thus avoiding the usual audit checks and balances.