Your network contains an Active Directory forest. The forest contains two domains named contoso.com and
corp.contoso.com. The forest contains four domain controllers. The domain controllers are configured as
shown in the following table.

All domain controllers are DNS servers.
In the corp.contoso.com domain, you plan to deploy a new domain controller named DC5.
You need to identify which domain controller must be online to ensure that DCS can be promoted successfully
to a domain controller.
Which domain controller should you identify?

A.
DC1

B.
DC2

C.
DC3

D.
DC4

Explanation:
A: Wrong Domain
B: Wrong Domain
C: Right domain, RID Master must be online
D: Right domain but Not needed to be online
Relative ID (RID) Master:
Allocates active and standby RID pools to replica domain controllers in the same domain.
(corp.contoso.com)
Must be online for newly promoted domain controllers to obtain a local RID pool that is required to advertise
orwhen existing domain controllers have to update their current or standby RID pool allocation.
The RID master is responsible for processing RID pool requests from all domain controllers in a
particulardomain.
When a DC creates a security principal object such as a user or group, it attaches a unique Security ID (SID)
tothe object.
This SID consists of a domain SID (the same for allSIDs created in a domain), and a relative ID (RID)that
isunique for each security principal SID created ina domain.
Each DC in a domain is allocated a pool of RIDs that it is allowed to assign to the security principals it creates.
When a DC’s allocated RID pool falls below a threshold, that DC issues a request for additional RIDs to
thedomain’s RID master.
The domain RID master responds to the request by retrieving RIDs from the domain’s unallocated RID pooland
assigns them to the pool of the requesting DC
At any one time, there can be only one domain controller acting as the RID master in the domain.

http://support.microsoft.com/kb/223346