Refer to the exhibits.

According to the FortiGate Administration Guide, “To use an external captive portal, you must configure a user group that uses the external captive portal as the authentication method and apply it to a firewall policy.” Therefore, option C is true because it will allow the wireless users to be redirected to the external captive portal URL when they try to access the Internet. Option A is false because disabling the user group from the SSID configuration will prevent the wireless users from being authenticated by the FortiGate device. Option B is false because enabling the captive-portal-exempt option in the firewall policy will bypass the captive portal authentication for the wireless users, which is not the desired outcome. Option D is false because including the wireless client subnet range in the Exempt Source section will also bypass the captive portal authentication for the wireless users, which is not the desired outcome.