Final answer:
An adverse opinion in an audit report is most likely when the financial statements significantly deviate from GAAP, indicating material misstatements and unreliable financial information.
Step-by-step explanation:
An adverse opinion is most likely to be included in an audit report when the financial statements depart from GAAP (Generally Accepted Accounting Principles). This type of opinion is given when the auditor concludes that the financial statements of an entity are materially misstated and do not accurately reflect the entity's financial position and results of operations in accordance with GAAP. It is a signal to users of the financial statements that the information may not be reliable.
In your specific options, an adverse opinion would not be appropriate for options a and b. A standard unmodified opinion indicates that the financial statements are presented fairly, and a public company's involvement does not inherently require an adverse opinion. However, if a client refuses to allow an auditor to perform a particular procedure (option c), it may lead to a different type of negative opinion known as a disclaimer of opinion, not necessarily an adverse opinion.