Codes and standards are established to promote minimum acceptable safety, performance, and reliability. NPPE guidance generally recognizes that deviation can be justified only with strong, defensible technical grounds and appropriate due diligence—such as when a standard does not apply to the situation (A), when there is credible evidence the standard is deficient or unsafe (B), or when new research or events demonstrate the standard is erroneous and a safer approach is required (D). In such cases, a professional must document the rationale, apply sound engineering/geoscience judgment, and often seek peer review or authority having jurisdiction input where appropriate. Option C is not a valid justification because cost, convenience, or perceived redundancy in the name of “duty to client” cannot override the duty to the public and the obligation to meet or exceed minimum safety requirements. Professionals must not reduce safety margins or ignore mandatory requirements simply to benefit a client’s interests. Therefore, C is the invalid reason.