Hi, when using the sharing option Anyone with the link can respond to get external responses, these are anonymous responses, there isn't any authentication, MFA and CA won't come into as the idea is for your respondents to get easy access to the form with this setting and be able to fill out the form. TLS encryption is used to protect the connection and information as it's submitted.
When using Only people in my organization can respond this should adhere to any CA or MFA policies, to access Microsoft Forms assuming a policy in place, for example, using Office 365 (preview), which as well as a Microsoft Forms also applies to all the core related services. In the M365 admin center you can also control external sharing centrally.
