According to the FortiGate Administration Guide, “To enable HTTPS authentication, you must enable HTTP redirect in the user authentication settings. This redirects HTTP requests to HTTPS. You must also update the captive portal URL to use HTTPS on both FortiGate and FortiAuthenticator.” Therefore, options B and D are true because they describe the changes that the administrator must make to enforce HTTPS authentication for the captive portal. Option A is false because creating a new SSID with the HTTPS captive portal URL is not required, as the existing SSID can be updated with the new URL. Option C is false because disabling HTTP administrative access on the guest SSID will not enforce HTTPS connection, but rather block HTTP connection.

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Using-secure-authentication-HTTPS-on-a-FortiGate/ta-p/192486