You are the Exchange administrator for contoso.
The network consists of a single Active Directory forest.
The forest contains the forest root domain contoso.com and one child domain japan.contoso.com.
User accounts and group accounts are contained in the child domain.
Management decides to deploy Exchange Server 2003 as the companywide messaging system.
You prepare the forest to support a new Exchange Server 2003 organization.
Replication completes normally.
You install the first Exchange Server 2003 system in the forest root domain.
You need to ensure that all user accounts can be mailbox-enabled.
What should you do?

A.
Run the setup /domainprep command in the forest root domain.

B.
Run the setup /domainprep command in the child domain.

C.
Install Active Directory Connector (ADC) on a domain controller in the forest root domain.

D.
Install Active Connector (ADC) on a domain controller in the child domain.

Explanation:
If you intend to mail enable users in any domain you must first "/domainprep" it. therefore B, if you "install Exchange 2003 in forest root then you must have run forestprep and domainprep prior to install so therefore B is your only answer.
Incorrect answers:
A: QUOTE: You install the first Exchange Server 2003 system in the forest root domain…this means you must have domainprep and forestprep’ed that root domain already!
C, D: ADC won’t help – noise – no mention of 5.5 GAL