What is the probable cause of this behaviour?

An architect has successfully configured SAML-BASED SSO for universal containers. SSO has been working for 3 months when Universal containers manually adds a batch of new users to salesforce. The new users receive an error from salesforce when trying to use SSO. Existing users are still able to successfully use SSO to access salesforce.

What is the probable cause of this behaviour?
A . The administrator forgot to reset the new user’s salesforce password.
B. The Federation ID field on the new user records is not correctly set
C. The my domain capability is not enabled on the new user’s profile.
D. The new users do not have the SSO permission enabled on their profiles.

Answer: B