Textual analysis is the best description because the auditor is reviewing bidding requirements across tender documents and identifying wording or specifications that appear to favor one supplier. Tender requirements are largely text-based, and unusual language, restrictive specifications, repeated phrases, or biased technical descriptions may indicate bid tailoring or bid-rigging risk. Diagnostic analysis explains why an event occurred, but the technique described is focused on analyzing written tender content. Predictive analysis forecasts future outcomes, and network analysis examines relationships between entities or nodes. Internal auditors may use textual analysis in procurement audits to detect suspicious specifications, duplicated wording, vendor-favored criteria, and manipulation of bid requirements. Therefore, Option C is correct.