What is preventing the administrator from enabling DHCP service on FortiGate's GUI?

Enabling the DHCP service on a FortiGate device requires the interface to have the appropriate role and configuration. If the role of the interface is restrictive, it means that the settings or permissions associated with that interface do not allow for DHCP services to be activated. For example, an interface set to a role that restricts traffic management or denies service configuration would prevent an administrator from enabling DHCP.

In this context, the interface's designated role must permit DHCP functionality. If the role is not configured correctly or is too restrictive, DHCP services will not be available in the GUI, making it impossible for the administrator to enable it through that interface.

The other options provide scenarios that, while plausible, do not specifically address the core issue at hand. If the GUI were malfunctioning or if DHCP could only be enabled via CLI, these would not directly involve the interface's role. Similarly, operation in monitoring mode typically implies a lack of ability to change configurations, but the primary determinant for enabling DHCP service is the assigned role of the interface.