You plan to deploy serverl.child.contoso.com as a read-only domain controller (RODC).
You run the adprep.exe /rodcprep command on DC3 and receive the following error message:

You need to identify what prevents you from successfully running Adprep /rodcprep on DC3.
What should you identify?

A.
The domain functional level of child.contoso.com isset to the wrong level.B.DC3 cannot connect to the domain naming master on DC1.

C.
The forest functional level is set to the wrong level.

D.
DC3 cannot connect to the infrastructure master onDC2.

Explanation:
Adprep could not contact a replica…
This problem occurs when the Adprep /rodcprep command tries to contact the infrastructure master for
each application partition in the forest.
Error message when you run the “Adprep /rodcprep” command in Windows Server 2008:
“Adprep could not contact a replica for partition DC=DomainDnsZones,DC=Contoso,DC=com”
https://support.microsoft.com/en-us/kb/949257