Final answer:
A reference to another auditor under U.S. auditing standards does not automatically result in a modification of the audit opinion; the principal auditor may still issue an unqualified opinion with a reference to the division of responsibility.
Step-by-step explanation:
Under U.S. auditing standards, when a reference to another auditor is made, it does not necessarily result in a modification of the audit opinion on its own. Instead, the principal auditor may decide to make reference to the other auditor's work and report in the form of a division of responsibility. This is typically included in the auditor's responsibility section of the audit report. However, this reference does not inherently affect the nature of the audit opinion, and the principal auditor may still issue an unqualified opinion. The circumstances under which a disclaimer, qualified, or adverse opinion may be issued are generally associated with situations where there is a limitation on the scope of the audit or the existence of substantive disagreements or uncertainties regarding the financial statements or their disclosures.