Final answer:
The AICPA principle that states the auditor must form an opinion on financial statements based on audit evidence is Principle 4, known as the Reporting Principle.
Step-by-step explanation:
According to the AICPA, the auditor needs to form an opinion on the financial statements based on an evaluation of the audit evidence obtained. The requirement for the auditor to form an opinion is stated in Principle 4 of the AICPA principles governing an audit conducted in accordance with Generally Accepted Auditing Standards (GAAS).
Principle 4, often referred to as the Reporting Principle, states that after conducting an audit in accordance with GAAS, the auditor should express an opinion regarding whether the financial statements are presented fairly, in all material respects, in accordance with the applicable financial reporting framework. This opinion is based on an evaluation of the audit evidence obtained during the performance of the audit.