Comprehensive and Detailed Explanation:

The most objective way to validate hours worked is to cross-check billed hours with independent evidence of presence — such as access logs (C) from security or entry systems. This provides direct evidence that contracted personnel were on-site.

Option A (due diligence review) focuses on vendor selection, not time validation.

Option B relies on vendor-supplied data, which may be manipulated.

Option D provides insights but is subjective.

According to IIA Standard 2310, evidence must be reliable and verifiable. Access logs provide such assurance, making Option C the strongest choice.