Choice "D" is correct. A lawyer's refusal to furnish the information requested in an inquiry letter would be a limitation on the scope of the audit sufficient to preclude an unmodified opinion.
Choice "c" is incorrect. An adverse opinion states that the financial statements do not present fairly the financial position or the results of operations or cash flows in conformity with generally accepted accounting principles. A lawyer's refusal to furnish the information requested in an inquiry letter does not necessarily mean that the financial statements are not fairly presented in conformity with GAAP.
Choice "b" is incorrect. A lawyer's refusal to furnish the information requested in an inquiry letter is not a reason to withdraw from the engagement, as long as the lawyer's refusal is not based on the client's request.
Choice "a" is incorrect. A lawyer's refusal to furnish the information requested in an inquiry letter is not considered to be a significant deficiency in internal control.