What is the best way to ensure that all data has been removed from a...

Allowing the cloud provider to manage your keys so that they have the ability to access and delete the data from the main and back-up storage.

Maintaining customer managed key management and revoking or deleting keys from the key management system to prevent the data from being accessed again.

Practice Integration of Duties (IOD) so that everyone is able to delete the encrypted data.

Keep the keys stored on the client side so that they are secure and so that the users have the ability to delete their own data.

Both B and D.