Based on the FortiOS 7.6 Authentication and User Group documentation, the correct answer is A.

Meaning of “Include in every user group” (FortiOS 7.6)

When configuring a RADIUS server on FortiGate, enabling Include in every user group has a very specific and documented effect:

The configured RADIUS server object is automatically added to all FortiGate user groups.

As a result, any user who successfully authenticates against that RADIUS server becomes a valid member of every FortiGate user group, unless additional group filtering (such as RADIUS attributes) is applied.

This simplifies configuration when the same external authentication source must be accepted across multiple firewall policies that reference different user groups.

This behavior is explicitly described in the FortiOS 7.6 Administrator Guide under RADIUS authentication servers and user groups.

Why Option A is Correct

FortiGate user groups can include:

Local users

LDAP servers

RADIUS servers

Enabling Include in every user group causes FortiGate to:

Insert the RADIUS server into all existing and future FortiGate user groups

Therefore, all users authenticating via this RADIUS server are implicitly allowed in every FortiGate user group.

This is exactly what option A describes.

Why the Other Options Are Incorrect

B: FortiGate does not push users or groups into the RADIUS server. Authentication is always initiated by FortiGate toward RADIUS.

C: FortiGate does not manage or modify RADIUS-side group definitions.

D: LDAP and RADIUS user groups are separate authentication mechanisms; this setting does not merge or affect LDAP groups.