Choice "C" is correct. When an auditor concludes there is substantial doubt about an entity's ability to continue as a going concern for a reasonable period of time, the auditor's responsibility is to consider the adequacy of disclosure about the entity's possible inability to continue as a going concern and include an emphasis-of-matter paragraph in the audit report.
Choice "a" is incorrect. The auditor would include an emphasis-of-matter paragraph following the unqualified opinion, or disclaim an opinion due to a material uncertainty. A qualified or adverse opinion is not appropriate for doubt about an entity's ability to continue as a going concern.
Choice "d" is incorrect. Management's accounting estimates are unrelated to going concern issues.
Choice "b" is incorrect. Going concern issues are considered prospectively. It is not appropriate to reissue a prior audit report if doubt arises about an entity's ability to continue in a future period.