To improve the detection of known attacks and behavioralIndicators of Compromise (IoCs), the best approach is tointegrate with an open-source threat intelligence feed. Threat intelligence feeds provide up-to-date information on known malicious IPs, domains, file hashes, and behavioral patterns that attackers use​.

Option A (randomly generating and storing hash values)is impractical, as there are an infinite number of possible files.

Option B (alerting on any system change)would lead to excessive noise and false positives, making the system difficult to manage.

Option D (manually adding signatures)is useful but is not scalable or as timely as an external intelligence feed​.

Thus, the correct answer isC, as integrating an open-source threat intelligence feed enhances the SIEM’s ability to detect and respond to real-world threats​.