Your company is planning to migrate to Microsoft Exchange Online. The company employs 1,000 people, each with a mailbox currently located on Exchange 2010
on-premises. You estimate that it will take a minimum of four weeks to migrate all mailboxes from on-premises Exchange to Exchange Online.
The company has the following migration requirements:
During the migration, do not change the existing Microsoft Outlook profiles and .ost files used by the employees.
Ensure that email messages sent between on-premises mailboxes and online mailboxes during the migration are secure.
Do not send email messages between on-premises mailboxes and online mailboxes over the Internet in plain text.
You need to select the migration strategy that meets the requirements.

Which migration strategy should you use?

A.
Cutover migration only

B.
IMAP migration only

C.
Remote move migration only

D.
Staged migration only

Explanation:
http://technet.microsoft.com/en-GB/library/jj863291(v=exchg.150).aspx
http://support.microsoft.com/kb/2798131/en-gb
http://technet.microsoft.com/en-GB/library/dn720476(v=exchg.150).aspx