Which of the following is a signature-based intrusion detection system (IDS) ?

RealSecure was an early commercial IDS/IPS product that used both signature-based and anomaly-based detection, but Snort is a more classic and prominent example of a pure signature-based NIDS.

StealthWatch (now Cisco Secure Network Analytics) is primarily a network flow monitoring and behavioral analytics tool that uses anomaly detection, not signature-based detection, to identify threats.

Tripwire is an integrity monitoring tool that detects changes to critical system files and configurations, rather than an intrusion detection system that analyzes network traffic based on signatures.

Snort is a well-known open-source network intrusion detection system (NIDS) that operates primarily on a signature-based detection model. It analyzes network packets in real-time and compares them against a vast collection of rules (signatures) that define known attack patterns, exploits, and policy violations.