Upgrading end-of-support operating systems is one of the most effective ways to reduce the attack surface. Unsupported OS versions no longer receive security patches, making them prime targets for attackers. Removing outdated software ensures that known vulnerabilities cannot be exploited. A (honeynet) is used for threat analysis, not reducing the attack surface. B (Group Policy) helps enforce security policies but does not address outdated vulnerabilities. C (High availability) focuses on uptime, not security risk reduction. Reference: CompTIA Security+ SY0-701 Official Study Guide, Security Architecture domain.