Final answer:
The correct action for an auditor who lacks independence with respect to a client is to issue a disclaimer of opinion, indicating the inability to provide assurance on the financial statements.
Step-by-step explanation:
When an auditor lacks independence with respect to a client, the appropriate action for the auditor to take is to issue a disclaimer of opinion. This is because the auditor's lack of independence casts doubt on the auditor's objectivity and, thus, their ability to perform an audit that adheres to the necessary professional standards. In this case, an adverse opinion is not appropriate as this indicates that the financial statements are materially misstated. A qualified opinion is also not suitable here, as it suggests that except for certain issues, the financial statements are fairly presented. An unqualified opinion, or clean opinion, is certainly not correct because it states that the financial statements are presented fairly in all material respects, which cannot be assured without auditor independence. Thus, option a) A disclaimer of opinion is the correct answer.