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:
Correct:
adprep /forestprep Must be run on the schema operations master for the forest > DC1
adprep /domainprep Must be run on the infrastructure operations master for the domain -> DC2
adprep /rodcprep Can be run from any computer. This command performs operations remotely. For the operations to complete successfully, the domain naming operations master for the forest and the infrastructure operations master for each application directory partition and each domain partition must be accessible. -> DC1 and DC2
https://technet.microsoft.com/en-us/library/dd464018(v=ws.10).aspx#BKMK_RunAdprep