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Question:

At Layer 5 of the OSI model, which of the following works?

A Circuit-level firewall
Explaination

This one, I must admit, is difficult. Yes, circuit-level firewalls function at Layer 5. Stateful firewalls can be considered to operate at Layer 5, however, their primary concentration is on Layers 3 and 4. Layer 7 is where the application operates.