File Integrity Monitoring (FIM) is specifically designed to detect and prevent unauthorized changes to critical system files, configuration files, registry entries, binaries, and logs. According to CompTIA Security+ SY0-701, FIM creates a cryptographic baseline (usually via hashing) of protected system files. Any attempt to modify, add, or delete protected files immediately triggers an alert, enabling rapid detection of tampering—whether caused by malware, insider threats, or misconfigurations.

NIDS (A) monitors network traffic, not system-level modifications. DLP (B) prevents unauthorized data exfiltration, not system-file tampering. NAC (C) controls device access to the network but does not protect system files.

FIM is a core tool for ensuring system integrity in compliance frameworks such as PCI-DSS, which explicitly requires organizations to monitor critical system files. By preventing unauthorized changes to system-level data and alerting administrators to suspicious activity, FIM provides a strong defensive mechanism against malware, ransomware, and configuration drift.

Thus, FIM is the correct answer.