Which command is used to connect to an SSH server as the 'stephen' user?

The command used to connect to an SSH server as the 'stephen' user is structured as "ssh username@host". In this case, the correct syntax includes specifying the username followed by the "@" symbol and then the host's IP address or hostname—'stephen' being the username and '172.16.10.123' being the IP address of the SSH server.

This format is a standard convention for SSH, enabling users to easily identify which account they are attempting to access on a remote machine. By using this specific syntax, it clearly indicates both the user and the target server in a single, concise command.

In comparison to the other choices, the additional commands either follow non-standard formats that aren't recognized by the SSH client or involve incorrect command usage.