In which of the following threats does an illicit denial of an event occur?

The correct answer is associated with the concept of repudiation, which refers to the denial of having performed a certain action or event. In the context of cybersecurity, repudiation typically occurs when an entity denies having performed a specific transaction or action, even when there is evidence to the contrary. This can lead to disputes and challenges in establishing accountability, particularly in environments where actions need to be traced back to individuals or systems.

Repudiation becomes a threat because it can undermine the integrity of the system by allowing malicious actors to deny harmful actions they have taken, or it can be a legitimate user denying an action that they actually performed. This lack of accountability can create security and compliance issues, especially in transactional environments or in situations requiring auditability. Hence, illicit denial of an event aligns directly with the concept of repudiation, making it the appropriate choice.

In contrast, while denial of service involves overwhelming a system to make it unavailable, and insiders typically refer to threats posed by individuals within an organization, these do not directly involve the denial of having committed an event. Insufficient due diligence relates more to failures in thorough examination or analysis of potential risks, and does not fit the specific scenario of an illicit denial of an action.