You have an Exchange Server organization that contains four servers. The servers are
configured as shown in the following table.
SiteA contains an IP gateway that uses a dial plan named Dialplan1. SiteB contains a Lync
Server 2013 server that uses a dial plan named Dialplan2. You plan to migrate all Unified
Messaging (UM) functionalities to Exchange Server 2013. You need to identify which tasks
must be performed to complete the migration. Which tasks should you identify? (To answer,
select the tasks that are required and not required in the answer area.)
Answer: 
Explanation:
Shouldn’t they all be required except EX1 & EX2 as it states you’re migrating to Exchange 2013 ?
On review I think the given answer is correct.
To integrate Exchange 2013 with Lync you need the dialplan associated to both 2013 servers (all CAS and MB server roles). Since the 2010 UM server will still be around during co-existence that one needs a dialplan too. So the answer is EX3/EX4 Dialplan1 – EX2 Dialplan2