What action must be taken after adding a security group to an aggregation security group that is currently in use?

When a security group is added to an aggregation security group that is actively in use, the appropriate action is to activate pending security policy changes. This step is crucial as it ensures that the updates regarding the changed security configuration are applied properly, allowing the system to recognize the new security group within the context of its policies.

Activating the pending changes allows all users to receive the appropriate access rights and permissions associated with the new security group's settings. Without this activation, the system would not implement the changes, potentially leaving users with outdated access rights or permissions, which could lead to security vulnerabilities or operational issues.

The other options involve actions that do not directly address the requirement for activating changes in the security configuration or could lead to unnecessary administrative overhead. Thus, focusing on the activation of pending changes is essential for maintaining effective security protocol in the system.