You have an Active Directory forest that contains three sites named Site1, Site2, and Site3. Each site contains two Exchange Server 2007 Client Access servers, two Mailbox servers, and two Hub Transport servers. All Exchange Server 2007 servers have Exchange Server 2007 Service Pack 1 (SP1) installed.

You need to ensure that you can deploy Exchange Server 2010 servers in Site1. You must achieve this goal by using the minimum amount of administrative effort.

What should you do?

A.
Upgrade all Client Access servers in the organization to Exchange Server 2007 Service Pack 2 (SP2).

B.
Upgrade all Exchange Server 2007 servers in Site1 to Exchange Server 2007 Service Pack 2 (SP2).

C.
Upgrade all Exchange Server 2007 servers in the organization to Exchange Server 2007 Service Pack 2 (SP2).

D.
Upgrade all Exchange Server 2007 servers in Site1 and all Client Access servers in the organization to Exchange Server 2007 Service Pack 2 (SP2).

Explanation:
The key to this question is the fact that you must use the minimum amount of administrative effort. Microsoft Technet therefore gives us the answer, as follows:

The Client Access server role can co-exist with Exchange 2007 Client Access servers. Before you upgrade the first Active Directory site, you must install Exchange 2007 Service Pack 2 (SP2) on all Exchange 2007 Client Access servers within your organization.

You can add an Exchange 2010 Hub Transport server to an existing Exchange organization after you successfully deploy Exchange 2010 Client Access servers. After you introduce Exchange 2010 Hub Transport servers to your Exchange 2007 environment, you still need to maintain your Exchange 2007 Hub Transport servers. Exchange 2010 Mailbox servers can only communicate with Exchange 2010 Hub Transport servers, and Exchange 2007 Mailbox servers can only communicate with Exchange 2007 Hub Transport servers. When a message is sent from a mailbox on an Exchange 2010 Mailbox server to a mailbox on an Exchange 2007 Mailbox server, the message is first submitted to the closest Exchange 2010 Hub Transport server in the site. This server then relays the message to an Exchange 2007 Hub Transport server in the same site, which finally delivers the message to the Exchange 2007 Mailbox server.

The Exchange 2010 Mailbox server role can co-exist with Exchange 2007 Mailbox servers.

You can replicate public folder data between Exchange 2010 and Exchange 2007 public folder databases.

Reference:

http://technet.microsoft.com/en-us/library/dd638158.aspx