What system detects unauthorized access and takes measures to prevent it?

The correct answer is the Intrusion Prevention System (IPS). This system plays a crucial role in cybersecurity by actively monitoring network traffic for suspicious activity and taking proactive measures to prevent unauthorized access. An IPS not only alerts administrators to potential threats but also responds automatically to mitigate these threats in real-time.

This is accomplished through a combination of known threat signatures, anomaly detection, and predefined policies. When an unauthorized attempt to access the network is detected, the IPS can block the traffic or drop the offending packets, thereby preventing potential breaches or harmful activities from occurring.

In contrast, an Intrusion Detection System (IDS) primarily focuses on monitoring and alerting about suspicious activities, without the capability to take immediate action to block or prevent those threats. Firewalls act as barriers that filter traffic based on security rules but do not necessarily detect and respond to specific unauthorized access attempts. Security Information and Event Management (SIEM) systems aggregate and analyze security data from across the network but are not designed to actively prevent unauthorized access like an IPS does. Thus, the IPS stands out as the most appropriate choice for detecting and preventing unauthorized access in a cybersecurity context.