Choice "B" is correct. External evidence received by the auditor is considered to be highly reliable and valid.
Choice "c" is incorrect. An invoice sent to the customer by the client is internal evidence, which is not as reliable as external evidence.
Choice "d" is incorrect. The customer's sales order is external evidence received and held by the client, which is not as valid as external evidence received directly by the auditor. In addition, a sales order alone is only evidence that an order was placed, not that it was filled or that a receivable exists.
Choice "a" is incorrect. An aging schedule is internal evidence, which is not as valid as external evidence.