What should you identify?

“User logon. In a forest that has more than one domain, two conditions require the global catalog during user authentication:

In a domain that operates at the Windows 2000 native domain functional level or higher, domain controllers must request universal group membership enumeration from a global catalog server.

When a user principal name (UPN) is used at logon and the forest has more than one domain, a global catalog server is required to resolve the name.”

http://technet.microsoft.com/en-us/library/cc728188(v=ws.10).aspx

So probably A.

“The PDC Emulator FSMO role owner performs the following functions:
Password changes performed by other DCs in the domain are replicated preferentially to the PDC emulator.
If a logon authentication fails at a given DC in a domain due to a bad password, the DC will forward the authentication request to the PDC emulator to validate the request against the most current password. If the PDC reports an invalid password to the DC, the DC will send back a bad password failure message to the user.
Account lockout is processed on the PDC emulator.”
http://msdn.microsoft.com/en-us/library/cc223752.aspx

So PDC Emulator is involved in UPDATING and CHANGED passwords, but if all users fail to log on, it’s unlikely it’s because they all changed their passwords simultaneously.

GC is needed on site 2.
“As a best practice, you should always configure the Active Directory domain controllers as the Global Catalog servers only when the domain is scattered in multiple sites. One Global Catalog server per site would be fine.” (http://www.tomshardware.co.uk/faq/id-1944457/make-windows-server-2012-global-catalog-server.html)
Answer: A

I agree with student that A is probably correct.

A is correct,NB. no password error is reported.

Supplies user principal name authentication.

A global catalog server resolves a user principal name (UPN) when the authenticating domain controller has no knowledge of the user account. For example, if a user’s account is located in sales1.cohovineyard.com and the user logs on with a UPN of [email protected] from a computer that is located in sales2.cohovineyard.com, the domain controller in sales2.cohovineyard.com cannot find the user’s account and it must contact a global catalog server to complete the logon process.

http://technet.microsoft.com/en-us/library/dd391870(v=ws.10).aspx

The PDC emulator processes password changes from earlier-version clients and other domain controllers on a best-effort basis; handles password authentication requests involving passwords that have recently changed and not yet been replicated throughout the domain; and, by default, synchronizes time. If this domain controller cannot connect to the PDC emulator, this domain controller cannot process authentication requests, it may not be able to synchronize time, and password updates cannot be replicated to it.

http://technet.microsoft.com/en-us/library/cc773108(v=ws.10).aspx

The PDC emulator master processes password changes from client computers and replicates these updates to all domain controllers throughout the domain. At any time, there can be only one domain controller acting as the PDC emulator master in each domain in the forest

The statement says that only users in Site2 fail to logon to child.contoso.com when the link fails. That domain spans Site1 and Site2. Users in Site1 apparently have no problem logging on. They have a PDC emulator. But since only one PDC emulator per domain is allowed, the answer must be the Global Catalog Server.

Pra mim resposta é emulador pdc. site 2 não possui nenhum serviço.