According to the Cisco Secure Email Threat Defense User Guide, the No Authentication option is only available if you are using a Cisco Secure Email Gateway (SEG) as your message source. This option allows visibility only, no remediation1.
The other options are not valid because:
A) Basic Authentication is not a visibility and remediation mode for Cisco Secure Email Threat Defense. It is a method of authenticating users with a username and password2.
C) Microsoft 365 Authentication is a visibility and remediation mode that allows you to use Microsoft 365 credentials to access Cisco Secure Email Threat Defense. It has two sub-options: Read/Write and Read. This mode is available for both Microsoft 365 and Gateway message sources1.
D) Cisco Security Cloud Sign On is not a visibility and remediation mode for Cisco Secure Email Threat Defense. It is a service that manages user authentication for Cisco security products, including Cisco Secure Email Threat Defense3.