The correct answer is C - Replacing the legacy firewalls with next-generation firewalls (NGFWs).
WGU Cybersecurity Architecture and Engineering (KFO1 / D488) outlines that NGFWs integrate advanced features like intrusion detection and prevention, application control, and real-time threat intelligence. They are designed to detect and alert on inbound threats as required by the mandate.
SIEM appliances (A) collect logs and correlate events but do not replace firewall functionality. Load balancers (B) manage traffic distribution, not threat detection. Reverse proxies (D) secure and balance traffic for web servers, not general firewall traffic.
Reference Extract from Study Guide:
"Next-generation firewalls (NGFWs) provide integrated intrusion detection and prevention capabilities, fulfilling modern security requirements for real-time threat monitoring and alerting."
- WGU Cybersecurity Architecture and Engineering (KFO1 / D488), Firewall Technologies and Modern Security Controls