You need to allow the user access only to the video file, and then revoke access once the user no longer needs it

You manage a collection of large video files that is stored in an Azure Storage account.
A user wants access to one of your video files within the next seven days.
You need to allow the user access only to the video file, and then revoke access once the
user no longer needs it.
What should you do?

A.
Give the user the secondary key for the storage account. Once the user is done with the
file, regenerate the secondary key.

B.
Create an Ad-Hoc Shared Access Signature for the Blob resource. Set the Shared
Access Signature to expire in seven days.

C.
Create an access policy on the container. Give the external user a Shared Access
Signature for the blob by using the policy. Once the user is done with the file, delete the
policy.

D.
Create an access policy on the blob. Give the external user access by using the policy.
Once the user is done with the file, delete the policy.

Explanation:
http://azure.microsoft.com/en-us/documentation/articles/storage-dotnet-sharedaccesssignature-part-1/