How can the new portal users register for portal access?

Answer options:

A.Register for a new local environment
B.Register using an external account
C.Register using the third-party account
D.Redeem invitation
E.Register using Azure AD account

Correct Answer: D 
By default, anybody can use the Power Apps portal as an unauthenticated user. If users want to register at the portal, there are two methods: Local authentication — creates a local identity with a username and password stored in a Dataverse database internally.
External authentication — uses credentials of the third-party identity providers, like OAuth2 (Microsoft, Twitter, Facebook, Google, etc.), OpenID (Azure AD, Azure AD B2C), and WS-Federation and SAML 2.0 (AD on-premises).
Portal administrators can enable/disable Identity providers and set them as default on the portal’s Authentication Settings panel. 
For each authenticated user, a portal creates a contact record in the Dataverse database. It also assigns a web role. The role defines a user`s access to portal resources. 
RentMe sends email invitations for portal registration. Only customers who received an email invitation can register and access the portal. They need to provide an invitation code (Number 3) from the email on the Sign-in (Number 1) screen and the Redeem Invitation (Number 2) tab. If the RentMe email includes an invitation code in the link`s body, the user can click a link, and the portal enables the normal procedure of creating a new login and password for the portal access. 

All other options are incorrect. 
For more information about Power Apps portal authentication, please visit the below URLs: 

https://docs.microsoft.com/en-us/learn/modules/introduction-power-apps-portals/06-overview-portals-security
https://docs.microsoft.com/en-us/powerapps/maker/portals/configure/set-authentication-identity#redeem-an-invitation