A content filter is the correct solution because it can block access to specific categories of websites, domains, URLs, or application types, including social media. Content filtering is commonly used to enforce acceptable use policies, reduce risk, improve productivity, and prevent users from accessing prohibited or unsafe web content. In this scenario, the requirement is to stop access to social media websites, so filtering web content by category or URL is the most direct control.

The CompTIA Network+ N10-009 objectives include network security concepts, access control, filtering, and security appliances or services. A content filter may be implemented through a secure web gateway, firewall, DNS filtering platform, proxy server, cloud security platform, or endpoint security solution.

Network Access Control, or NAC, controls whether devices are allowed to connect to the network based on identity, posture, or policy. A screened subnet, also called a DMZ, is used to isolate public-facing services. A VPN provides encrypted connectivity but does not inherently block social media access. Therefore, content filtering is the most appropriate answer.