Answer: The correct answer is "D. It begins when the engagement letter is signed and continues until the report for the third year is issued unless the relationship is terminated sooner.".
Explanation: The statement "It begins when the engagement letter is signed and continues until the report for the third year is issued unless the relationship is terminated sooner." best describes the period of the professional engagement as it applies to a three-year engagement to audit client's financial statements since this type of professional commitment begins with the signing of the document that formalizes the commitment and is in force until the issuance of the last report unless the relationship is resolved beforehand by another circumstance.