The correct answer is Network Intrusion Detection System.

A Network Intrusion Detection System passively monitors network traffic to identify suspicious patterns and it generates alerts when it detects signatures or anomalies. It is typically deployed on a network segment or using a span or tap so it can inspect copies of traffic without interfering with normal flow. Because it focuses on detection and alerting rather than active blocking it directly matches the requirement to monitor traffic and produce alerts.

A Network Intrusion Detection System often combines signature based detection and behavioral analysis and it can forward alerts and context to a SIEM or security operations team for investigation and response. This makes it suitable for continuous monitoring across a network and for notifying analysts about potential intrusions.

Firewall enforces access control between networks by allowing or denying traffic based on configured rules and it is not primarily used as a detailed monitoring and alerting system for suspicious activity. Traditional firewalls focus on permitting or blocking flows rather than producing investigative alerts.

Web Application Firewall protects web applications by inspecting HTTP and HTTPS traffic for application layer attacks such as SQL injection and cross site scripting. It is specialized for web traffic and it does not provide the broad network traffic monitoring and alerting described in the question.

Intrusion Prevention System can detect malicious activity but it is designed to take active measures to block or prevent traffic and it typically operates inline. Since the question emphasizes monitoring and producing alerts the passive detection and alerting role of a network intrusion detection system is a better fit than an IPS which focuses on prevention.

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