Email Proof is the User access and sharing feature that would be used for the following use case: it is the first time a user logs in and they need to be sent an email with a way to authenticate their email. Email Proof is a security feature that requires users to verify their email address when they log in for the first time or change their email address. 2 FA, Single Sign-On, and Login Link are not User access and sharing features that would be used for this use case. References: https://help.salesforce.com/s/articleView?id=sf.
security_email_proof.htm&type=5