Comprehensive and Detailed Explanation:
While professional experience and judgment are important, conclusions in internal audit must be supported by objective, sufficient, and reliable evidence (Standard 2310). Merely believing that bid prices are too high is not enough. The auditor must provide substantiated and comparative evidence (C) - such as market benchmarks, independent price analyses, or comparisons with similar tenders - to justify the conclusion. A description of policies (A) or tender process summaries (B) provide context but do not prove overpricing. An impact analysis (D) highlights consequences but does not establish whether prices were indeed unreasonable.
Therefore, the correct requirement is to gather and document comparative evidence that supports the auditor's professional judgment.