A vulnerability scan is the most effective method for identifying legacy systems within an environment. Vulnerability scanners assess hosts for outdated operating systems, unsupported software versions, missing patches, deprecated services, and known Common Vulnerabilities and Exposures (CVEs). CompTIA Security+ SY0-701 highlights vulnerability scanning as a foundational security operation used to gain visibility into system age, patch status, and configuration weaknesses.

Legacy systems often stand out in scan results because they run end-of-life operating systems, use deprecated protocols, or lack current security updates. These indicators allow security teams to quickly flag systems that require isolation, compensating controls, or replacement.

Bug bounty programs (A) rely on external researchers and are not designed to inventory internal assets. Package monitoring (C) tracks software behavior and changes but does not identify system age or support status. Dynamic analysis (D) evaluates running applications for vulnerabilities, not infrastructure lifecycle status.

Because vulnerability scans provide broad visibility into system versions and supportability, the correct answer is B: Vulnerability scan.