In which situation is it BEST to use a detective control type for mitigation?

Using a detective control type for mitigation is most appropriate in scenarios where the focus is on identifying and responding to incidents after they occur, rather than preventing them outright. In the context given, purchasing an Intrusion Prevention System (IPS) that is designed to monitor traffic only fits this definition.

The IPS functions as a detective control because it analyzes network traffic in real-time to identify potential threats and anomalies. While it doesn't actively block malicious activity (which would classify it as a preventative control), it alerts administrators to suspicious activities, enabling them to investigate and respond accordingly. This situational context is crucial for organizations that prioritize monitoring for threats and forensics in their security posture.

In contrast, the other scenarios focus more on proactive mitigation strategies. For instance, a network load balancer improves availability by distributing traffic to multiple servers, a backup solution ensures data recoverability in the event of loss (disaster recovery being a proactive approach), and an application-level firewall isolates traffic to block unwanted access rather than primarily detecting issues. Therefore, the chosen approach highlights the critical nature of detective controls in strengthening an organization's awareness and response capabilities regarding security incidents.