Which of the following can be best used to discover a company's publicly available breach...

OSINT (Open Source Intelligence) involves collecting publicly available information, including data breaches, leaked credentials, and other exposed company data found online or on the dark web. OSINT tools can discover if a company’s information has been compromised publicly.

SIEM (Security Information and Event Management) monitors internal logs and events but does not collect external breach info. CVE (Common Vulnerabilities and Exposures) is a database of known software vulnerabilities, not breach data. CVSS (Common Vulnerability Scoring System) rates vulnerabilities' severity.

OSINT is a key technique in the Threats and Vulnerabilities domain for external threat intelligence gathering【6:Chapter 2†CompTIA Security+ Study Guide】.