Choice "C" is correct. The practitioner issuing a covered opinion must be knowledgeable in all aspects of federal tax law relevant to the opinion being rendered and may rely on the opinion of other practitioners for part(s) of the opinion, with identification of the other practitioner's opinion and conclusion.Choice "a" is incorrect. The practitioner issuing a covered opinion must be knowledgeable in all aspects of federal tax law relevant to the opinion being rendered, not all aspects of federal tax law, and may rely on the opinion of other practitioners for parts of the opinion.Choice "d" is incorrect. The practitioner issuing a covered opinion must be knowledgeable in all aspects of federal tax law relevant to the opinion being rendered and may rely on the opinion of other practitioners for part(s) of the opinion. Identification should be made of the other practitioner's opinion and conclusion.Choice "b" is incorrect. A written advice subject to contractual protection is not one where the practitioner has a written (or oral) contract to issue the written advice. It is a written advice where (1) the fee is either fully or partially refundable if all or a part of the intended tax consequences from the matters addressed in the written advice are not sustained or (2) the fee is contingent on the taxpayer's realization of tax benefits from the transaction.