Choice "C" is correct. When audit evidence can be obtained from independent sources outside an entity, it provides greater assurance of reliability for the purposes of an independent audit than does evidence secured solely within the entity. While the bank statement was obtained from the client, it is still more persuasive than any of the other three items because it was not prepared by the client.
Choice "b" is incorrect. Prenumbered client purchase orders are client-generated documents; as such, they are not as persuasive as externally generated evidence received through a client.
Choice "a" is incorrect. Client work sheets supporting cost allocations are client-generated documents; as such, they are not as persuasive as externally generated evidence received through a client.
Choice "d" is incorrect. The client representation letter is a client-generated document; as such, it is not as persuasive as externally generated evidence received through a client.