Which of the following is NOT a commonly used SIEM tool?

Apache Log4j is not a commonly used Security Information and Event Management (SIEM) tool. Instead, it is a Java-based logging utility that is primarily used for logging application data. While logging is an essential component of security monitoring, Log4j itself does not provide the comprehensive analysis, event correlation, and security alerting functionalities that define SIEM systems.

In contrast, IBM QRadar, ArcSight, and Splunk are all prominent SIEM tools that are designed to aggregate, analyze, and manage security data from various sources. They facilitate real-time monitoring and analysis of security incidents, providing organizations with the ability to detect and respond to threats effectively. By using advanced analytics, these tools help security teams identify patterns and anomalies in data that might indicate potential security breaches or cyber threats.