Choice "D" is correct. The auditor should consider the types of information respondents will be readily able to confirm. For instance, some accounting systems facilitate the confirmation of single transactions rather than entire balances. In such cases, the auditor might consider including a client-prepared statement of account showing details of the customer's account balance being confirmed. By making it easier for customers to determine which items are included in the balance being confirmed, the auditor also makes it more likely that those customers will respond.
Choice "b" is incorrect. Restricting the selection of accounts to be confirmed to those customers with large balances doesn't improve response rates, as customers with large balances are not necessarily more likely to respond than customers with small balances.
Choice "a" is incorrect. Explaining to customers that discrepancies will be investigated by an independent third party would not necessarily encourage them to respond, as they might be reluctant to set off this investigation.
Choice "c" is incorrect. Responses received by fax should be verified by calling the senders and requesting that the original confirmations be mailed back. Asking customers to respond by fax might actually reduce the response rate for receiving the original confirmation, since customers may decide that since they already sent the fax, they don't need to send the original back as well.