A Web Application Firewall (WAF) is specifically designed to monitor, filter, and block data packets as they travel to and from a web application or website. It provides protection from a variety of application-layer attacks, such as cross-site scripting (XSS), SQL injection, and others, by inspecting HTTP traffic. Unlike traditional firewalls, a WAF works at the application layer and is able to understand and analyze the content of web traffic, which is why it is the best solution in this scenario. Proxy servers can provide anonymity and may protect against some threats, but they don't offer the same level of protection for web applications. Network Access Control (NAC) manages access to network resources by devices and users, which is not specifically related to web application threats. Data Loss Prevention (DLP) systems monitor, detect, and block data breaches/data exfiltration transmissions, which is a different focus from what is required in the scenario.