B. This option places the RADIUS server, and all users who can authenticate against that server, into every FortiGate user group.
FortiGate_Security_7.2_Study_Guide page 146
The Include in every User Group option adds the RADIUS server and all users that can authenticate against it, to every user group created on FortiGate. So, you should enable this option only in very specific scenarios (for example, when only administrators can authenticate against the RADIUS server and policies are ordered from least restrictive to most restrictive).