Your company has an Active Directory domain.
The company runs Remote Desktop Services.
Standard users who connect to the Remote Desktop Session Host Server are in an organizational unit (OU) named OU1.
Administrative users are in OU1.
No other users connect to the Remote Desktop Session Host Server.
You need to ensure that only members of OU1 can run the Remote Desktop Protocol files.
What should you do?

A.
Create a Group Policy object (GPO) that configures the Allow .rdp files from unknown publishers policy setting in the Remote Desktop Client Connection template to Disabled. Apply the GPO to OU1.

B.
Create a Group Policy object (GPO) that configures the Allow .rdp files from valid publishers and user’s default .rdp settings policy setting in the Remote Desktop Client Connection template to Disabled. Apply the GPO to OU1.

C.
Create a Group Policy object (GPO) that configures the Allow .rdp files from valid publishers and user’s default .rdp settings policy setting in the Remote Desktop Client Connection template to Enabled. Apply the GPO to OU1.

D.
Create a Group Policy object (GPO) that configures the Specify SHA1 thumbprints of certificates representing trusted .rdp publishers policy setting in the Remote Desktop Client Connection template to Enabled. Apply the GPO to OU1.

Explanation:
Allow .rdp files from valid publishers and user�s default .rdp settings
This policy setting allows you to specify whether users can run Remote Desktop Protocol (.rdp) files from a publisher that signed the file with a valid certificate. A valid certificate is one that is issued by an authority recognized by the client, such as the issuers in the client’s Third-Party Root Certification Authorities certificate store. This policy setting also controls whether the user can start an RDP session by using default .rdp settings (for example, when a user directly opens the Remote Desktop Connection [RDC] client without specifying an .rdp file).
If you enable or do not configure this policy setting, users can run .rdp files that are signed with a valid certificate. Users can also start an RDP session with default .rdp settings by directly opening the RDC client. When a user starts an RDP session, the user is asked to confirm whether they want to connect.
If you disable this policy setting, users cannot run .rdp files that are signed with a valid certificate. Additionally, users cannot start an RDP session by directly opening the RDC client and specifying the remote computer name. When a user tries to start an RDP session, the user receives a message that the publisher has been blocked.
Source: http://technet.microsoft.com/en-us/library/ee791914(WS.10).aspx