What should be enabled to reactivate unknown Agents?

To reactivate unknown Agents in Trend Micro Deep Security, enabling Agent-Initiated Activation and the Reactivate unknown Agents feature is essential. This functionality allows agents that are unable to communicate with the Deep Security Manager due to issues like network changes or configuration problems to reconnect and re-establish their status.

When this setting is activated, the agents can automatically reach out to the manager and restore their connection without requiring manual intervention. This automated process is crucial for maintaining continuous protection and management of your environment, particularly in dynamic settings where agents may frequently go offline or become "unknown."

The other options do not specifically address the need to reactivate unknown agents. Background updates focus on keeping the agent's definitions and software current, while manual activation would require a hands-on approach to reestablish each agent's connection, which is less efficient. Scheduled scanning pertains to the regular scanning of systems for threats rather than the activation or reactivation of agents. Thus, for efficient management of agents and ensuring they remain integrated within the security framework, the option that supports automatic reactivation is the most effective and necessary choice.