Isolating affected systems limits the damage and prevents the incident from impacting other parts of the organization.
"During containment, the primary objective is to isolate affected systems to prevent further damage."
- CISM Review Manual 15th Edition, Chapter 4: Incident Management, Section: Incident Response Process*

This approach is the best because it ensures that users have the minimum level of access required to perform their job functions, which reduces the risk of unauthorized access or misuse of data. User roles are defined based on the business needs and responsibilities of the users, and they can be easily managed and audited.
References: The CISM Review Manual 2023 states that "the data owner is responsible for defining the access privileges for each user role" and that "the data owner should ensure that the principle of least privilege is applied to all users" (p. 82). The CISM Review Questions, Answers & Explanations Manual 2023 also provides the following rationale for this answer: "Defining access privileges based on user roles is the best approach because it allows the data owner to assign the minimum level of access required for each role and to review and update the roles periodically" (p. 23).

Recovery time objectives (RTOs) are the maximum acceptable time that an organization can be offline or unavailable after a disruption. RTOs are important to maintain integration among the incident response plan, business continuity plan (BCP), and disaster recovery plan (DRP) because they help align the recovery goals and strategies of each plan. By defining clear and realistic RTOs, an organization can ensure that its IT infrastructure and systems are restored as quickly as possible after a disaster, minimizing the impact on business operations and customer satisfaction.
References = CISM Manual, Chapter 6: Incident Response Planning, Section 6.2: Recovery Time Objectives (RTOs), page 971
1: https://store.isaca.org/s/store#/store/browse/cat/a2D4w00000Ac6NNEAZ/tiles

The first thing that should be done when a SIEM flags a potential event is A. Validate the event is not a false positive. This is because a false positive is an event that is incorrectly identified as malicious or suspicious by the SIEM, when in fact it is benign or normal. False positives can waste the time and resources of the security team, and reduce the trust and confidence in the SIEM system. Therefore, it is important to verify the accuracy and validity of the event before initiating any further actions, such as incident response, escalation, or compensating controls. Validation can be done by analyzing the event data, comparing it with the baseline or normal behavior, and checking for any anomalies or indicators of compromise.
A false positive is an event that is incorrectly identified as malicious or suspicious by the SIEM, when in fact it is benign or normal. Validation can be done by analyzing the event data, comparing it with the baseline or normal behavior, and checking for any anomalies or indicators of compromise. (From CISM Manual or related resources) References = CISM Review Manual 15th Edition, Chapter 4, Section 4.2.1, page 2091; CISM Review Questions, Answers & Explanations Manual 9th Edition, Question 72, page 19