Final answer:
To implement User Login for public sites in Salesforce, Username and Password authentication is typically used. SSO and OAuth are more for system integrations, while 2FA adds security to existing authentication.
Step-by-step explanation:
In Salesforce, to implement User Login (Authentication) for public sites, typically the Username and Password authentication (C) method is used.
Public sites may not use Salesforce's built-in user authentication as they are accessible to users without logging in.
However, if you need to secure parts of your site and require users to log in, you could potentially use a custom authentication method involving username and password checks against a contact or user stored in Salesforce.
Single Sign-On (SSO) (A) and OAuth (B) are largely for integrating Salesforce with external systems or allowing users who are already authenticated in another system to access Salesforce without logging in again.
These methods aren't typically used for authenticating public site users. Furthermore, Two-Factor Authentication (2FA) (D) is an additional security layer for already authenticated users.