Final answer:
When an auditor's opinion is supported by appraisers' conclusions documented in their files, it is considered the least persuasive form of support. Other forms of support hold greater weight in validating the audit opinion.
Step-by-step explanation:
The least persuasive documentation in support of an auditor's opinion among the options provided is c. Notation of appraisers' conclusions documented in the auditor's files. Appraisers' conclusions are not considered reliable evidence as they are subjective and can vary from person to person. In contrast, options a, b, and d provide more objective and verifiable evidence such as physical inventory counts, inferences from ratios and trends, and confirmation responses.