If an auditor implemented a control, they would later be reviewing their own work, which creates a self- review threat and compromises independence. Participating in the project without operational responsibilities (B) or providing advice (C) is acceptable as long as the auditor does not take ownership of decisions.
Designing audit modules (D) is also permissible since they are for audit use and do not affect operational processes. ISACA's Code of Professional Ethics and IS Audit Standards emphasize independence and objectivity as fundamental requirements to maintain credibility and avoid conflicts of interest.
References (ISACA): ISACA Standards for IS Audit and Assurance; ISACA Code of Professional Ethics.