Final answer:
The WAF (Web Application Firewall) operates on the highest layer of the OSI model and is the most effective at blocking application attacks.
Step-by-step explanation:
The correct answer is WAF (Web Application Firewall). A WAF operates on the highest layer of the OSI model, the Application layer. It is specifically designed to protect web applications from various types of attacks, including application-layer attacks. By analyzing and filtering HTTP traffic, a WAF can proactively block common application attacks such as SQL injection, cross-site scripting (XSS), and remote file inclusion.