HOTSPOT
Your network contains an Active Directory forest named contoso.com. The forest contains one domain.
All domain controllers run Windows Server 2012. The functional level of the forest and the domain is
Windows Server 2012.
The domain contains three domain controllers. The domain controllers are configured as shown in the
following table.
In the forest, you plan to add a new domain controller that runs Windows Server 2012 R2.
You need to prepare the environment before you add the new domain controller.
Which domain controllers must be available to run each command? To answer, select the appropriate
domain controllers in the answer area.
Answer: 
Explanation:
* adprep /forestprep
Must be run on the schema operations master for the forest (here DC1).
Once for the entire forest
* adprep /domainprep
Must be run on the infrastructure operations (DC2) master for the domain.
* adprep /rodcprepCan be run from any computer. This command performs operations remotely. For the operations to
complete successfully, the domain naming operations master (DC1) for the forest and the infrastructure
operations master (DC2) for each application directory partition and each domain partition must be
accessible.
Running Adprep.exe
https://technet.microsoft.com/en-us/library/dd464018(v=ws.10).aspx