Final answer:
HTTP, FTP, and Telnet all operate at the Application Layer (Layer 7) of the OSI model, which is the highest layer for these network protocols.
Step-by-step explanation:
The highest layer at which HTTP, FTP, and Telnet protocols operate is Layer 7, which is known as the Application Layer in the OSI model (Open Systems Interconnection model).
This layer supports application and end-user processes, meaning it provides data to the user and ensures that communication is possible over the network. Protocols like HTTP (Hypertext Transfer Protocol), FTP (File Transfer Protocol), and Telnet operate at this layer to provide the necessary functions for different applications.