Explanation/Reference:
Explanation
C: You only need to test and enable a mailbox if you're using server-side synchronization as your synchronization method. You don't need to test and enable a mailbox if you're using CRM for Outlook or the Email Router for synchronization purposes.
Every user in CRM gets a CRM mailbox by default. After your mailbox is set up, to synchronize it with an email server (Exchange or POP3) using server-side synchronization, you have to click the Test & Enable Mailbox button to make sure the mailbox is linked and configured correctly.
A: You can use mailbox monitoring to poll one or more mailboxes for incoming email messages, and then determine what actions Microsoft Dynamics CRM will take based on the email message, such as create or update records in the system. You can configure server-side synchronization or the Email Router to monitor either of the following:
A forward mailbox. This is a single, central mailbox.

The mailbox for each user or queue.

If you administer an organization that has to monitor a large number of mailboxes, you should consider using a forward mailbox to reduce the administrative effort. Monitoring many mailboxes can sometimes require maintaining access credentials in many incoming configuration profiles.
References: https://blogs.msdn.microsoft.com/crm/2015/08/31/test-and-enable-mailboxes-in-microsoft- dynamics-crm-2015/
https://technet.microsoft.com/en-us/library/hh699793.aspx