Client acceptance (A) is a critical post-report delivery activity that involves the client formally accepting the findings and conclusions of a penetration assessment report. This process usually includes a review of the findings by the client, discussions about the impact, and agreement on the accuracy and relevance of the reported vulnerabilities and issues. Ensuring client acceptance soon after the delivery of the report can prevent scenarios where the validity of findings is debated long after the assessment, as in the case described.

Data destruction process (B), attestation of findings (C), and lessons learned (D) are also important aspects of a penetration testing engagement, but they do not directly address the issue of the client disputing the findings well after the report has been delivered. Client acceptance ensures both parties are in agreement on the outcomes of the assessment, minimizing disputes about the findings later on.