The MOST significant limitation of vulnerability scanning is the fact that modern scanners only detect known vulnerabilities. This is because vulnerability scanners rely on databases or repositories of known vulnerabilities, such as CVE (Common Vulnerabilities and Exposures), to compare and identify the weaknesses or flaws in systems or applications. Vulnerability scanners cannot detect unknown vulnerabilities, such as zero-day vulnerabilities, that have not been reported or disclosed yet, and may be exploited by attackers before they are patched or fixed. The other options are not the most significant limitation of vulnerability scanning, because they either involve detecting common (A), unknown (B), or zero-day (D) vulnerabilities, which are not the capabilities or limitations of modern scanners.