Your network contains the servers configured as shown in the following table:

You run the New-DatabaseAvailabilityGroup cmdlet and you receive the following error message:
The task was unable to create the default witness directory on server
FS01. Please manually specify a witness directory.
+ CategoryInfo: InvalidArgument:(:)[New-DatabaseAvailabilityGroup],
DagFswUnableTo … ectoryException
+ FullyQualifiedErrorId: [Server=EX01, RequestId-81b106a2-21e7-4ac3-
8a23-e2e13e54a9fa, TimeStamp=11/13/2015 3:09:02 PM]
[FailureCategory=CmdletDagFswUnableToBindWitnessDirectoryException]166B
6E12,Microsoft.Exchange.Management.SystemConfigurationTasks.NewDatabase
AvailabilityGroup
+ PSComputerName: EX01.contoso.com
You need to ensure that you can create the database availability group (DAG) successfully by using
the NewDatabaseAvailabilityGroup cmdlet. What should you do?

A.
Add the Exchange Trusted Subsystem group to the local Administrators group on FS01.

B.
Add the Delegated Setup role group to the local Administrators group on FS01.

C.
Install the Failover Clustering feature on FS01.

D.
Install the Remote Server Administration Tools feature on FS01.

Explanation:
http://exchangeserverpro.com/using-a-non-exchange-server-as-an-exchange-2013-dag-fileshare-witness/