Choice "B" is correct. A disclaimer of opinion means that the auditor was unable to obtain sufficient appropriate audit evidence to provide a reasonable basis for an opinion, thus, no opinion is expressed. An unjustified change in accounting principle could result in a material misstatement of the financial statements that would result in a qualified or adverse opinion, not a disclaimer of opinion.
Choice "d" is incorrect. Inability to determine amounts associated with an employee fraud scheme is a scope limitation that may result in a disclaimer of opinion.
Choice "a" is incorrect. Refusal by the client to permit the auditor to confirm accounts receivable is a scope limitation that may result in a disclaimer of opinion.
Choice "c" is incorrect. Refusal of management to sign a management representation letter casts doubt on the audit evidence gathered and is a scope limitation that would likely result in a disclaimer of opinion.