An unknown source has attacked an organization’s network multiple times.

An Intrusion Prevention System (IPS) is the most effective addition when an organization already has a firewall but continues to face repeated external attacks. Security+ SY0-701 states that an IPS operates inline and automatically blocks malicious traffic in real time based on signatures, anomaly behavior, or heuristics. Whereas a firewall filters traffic by rules, an IPS detects and prevents deeper-level threats such as exploits, malware, and command-and-control attempts.

A UTM (C) includes IPS features, but it is typically used to replace a firewall with an all-in-one appliance. The question states the organization already has a firewall, so the most efficient addition is a standalone IPS. A SIEM (A) aggregates and analyzes logs but does not block attacks. A load balancer (B) distributes traffic for performance—not security.

Thus, the best item to stop active inbound attacks is D: IPS.