Which system detects malicious activity across a network but does not block the activity?

The correct choice for this question is an Intrusion Detection System (IDS). An IDS is designed to monitor network traffic for suspicious activities and potential threats. Its primary function is to analyze traffic, identify abnormal patterns, and report those findings to administrators or threat response teams.

The key characteristic of an IDS is that it does not actively block or prevent malicious activity; rather, it operates in a passive mode. When a potential threat is detected, alerts are generated so that human operators can take appropriate actions, such as investigating and responding to the threat.

In contrast, an Intrusion Prevention System (IPS) actively prevents detected threats by blocking them in real-time, which is a crucial distinction. A Network Address Prevention System is not a standard term widely recognized in the industry, leading to confusion regarding its role compared to established systems. Finally, a Network Access Control System focuses primarily on regulating user access to network resources rather than detecting and reporting malicious activities. Thus, the vital differentiator is that the Intrusion Detection System alerts to potential threats without blocking them, making it the correct answer.