The primary reason to establish the root cause of an IT security incident is to avoid recurrence of the incident. By identifying and addressing the underlying cause of the incident, the organization can prevent or reduce the likelihood of similar incidents in the future. This can also help to improve the security posture and resilience of the organization. The other options are not the primary reason, but they may be secondary or tertiary reasons. Preparing a report for senior management is an important step in communicating the incident and its impact, but it does not address the root cause. Assigning responsibility and accountability for the incident is a way to ensure that the appropriate actions are taken to remediate the incident and prevent recurrence, but it is not the reason to establish the root cause. Updating the risk register is a part of the risk management process, but it does not necessarily prevent recurrence of the incident. References = Risk and Information Systems Control Study Manual, 7th Edition, Chapter 4: Risk Response and Reporting, Section 4.3: Incident Management, p. 223-224.