Choice "A" is correct. This is a scope limitation, which results in either a qualified opinion or a disclaimer of opinion. (Note that the question does not say that management's estimate is unreasonable, just that the auditor is unable to determine whether the estimate is reasonable.)
Choice "d" is incorrect. An adverse opinion is not expressed for scope limitations.
Choices "c" and "b" are incorrect. A scope limitation believed to have a material effect would not result in an unmodified opinion.
