Which command is used to restart a BGP session between two peers?

The command to restart a BGP session between two peers is "execute router clear bgp ?". This command is specifically designed for managing the Border Gateway Protocol (BGP) sessions. When you run this command, it allows the user to clear specific BGP sessions or all BGP sessions, which effectively restarts them and can help in resolving issues such as session hangs or route updates not propagating as expected.

The BGP protocol relies on maintaining a stable connection between peers for route exchange, and clearing the session can force a fresh connection, helping to re-establish the communication necessary for routing information. This command can be used in various contexts, whether to clear a specific BGP peer or all peers connected to the router, making it a versatile tool for network administrators dealing with BGP configurations.

The other options listed do not pertain to managing BGP sessions. For instance, commands related to debug or Fortiguard services are unrelated to BGP session management, making them unsuitable for restarting BGP peer connections. This specificity in purpose and functionality is what establishes the chosen command as the correct answer.