Choice "D" is correct. If the results of an analytical procedure disclose unexpected differences, the auditor should consider that the financial statements may contain a material misstatement.
Choice "c" is incorrect. Management's failure to disclose to the auditor that it consulted with other accountants about significant accounting matters would not heighten the auditor's concern about whether the financial statements contain material misstatements.
Choice "b" is incorrect. The management representation letter is typically not signed until the last day of the auditor's field work.
Choice "a" is incorrect. If audit trails of computer-generated transactions exist only for a short time, the auditor should adjust the timing of his/her audit procedures accordingly, but this would not heighten the auditor's concern about whether the financial statements contain material misstatements.