Choice "D" is correct. A CPA must maintain objectivity and integrity in the performance of any professional service.
Choice "a" is incorrect. Independence is only required for audits and other attestation services. A CPA who is not in public practice would not perform audits or attestation services.
Choice "b" is incorrect. Contingent fees are not rules of conduct. They are fees based on a specific finding or result, and they are often prohibited for attest engagements.
Choice "c" is incorrect. Commissions are prohibited if the CPA is performing an audit or review, an examination of prospective financial information or a compilation when the member does not disclose independence. These prohibitions are not applicable to a CPA who is not in public practice.